Move uninhabited tests into subdirectory.

This commit just tries to tidy up a little.

src/test/ui/rfc-2008-non-exhaustive/uninhabited/coercions.stderr

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/uninhabited.rs:23:5
+  --> $DIR/coercions.rs:23:5
    |
 LL | fn cannot_coerce_empty_enum_to_anything(x: UninhabitedEnum) -> A {
    |                                                                - expected `A` because of return type
@@ -10,7 +10,7 @@ LL |     x
               found type `uninhabited::UninhabitedEnum`
 
 error[E0308]: mismatched types
-  --> $DIR/uninhabited.rs:27:5
+  --> $DIR/coercions.rs:27:5
    |
 LL | fn cannot_coerce_empty_tuple_struct_to_anything(x: UninhabitedTupleStruct) -> A {
    |                                                                               - expected `A` because of return type
@@ -21,7 +21,7 @@ LL |     x
               found type `uninhabited::UninhabitedTupleStruct`
 
 error[E0308]: mismatched types
-  --> $DIR/uninhabited.rs:31:5
+  --> $DIR/coercions.rs:31:5
    |
 LL | fn cannot_coerce_empty_struct_to_anything(x: UninhabitedStruct) -> A {
    |                                                                    - expected `A` because of return type
@@ -32,7 +32,7 @@ LL |     x
               found type `uninhabited::UninhabitedStruct`
 
 error[E0308]: mismatched types
-  --> $DIR/uninhabited.rs:35:5
+  --> $DIR/coercions.rs:35:5
    |
 LL | fn cannot_coerce_enum_with_empty_variants_to_anything(x: UninhabitedVariants) -> A {
    |                                                                                  - expected `A` because of return type

src/test/ui/rfc-2008-non-exhaustive/uninhabited/coercions_same_crate.stderr

@@ -1,5 +1,5 @@
 error[E0308]: mismatched types
-  --> $DIR/uninhabited_same_crate.rs:31:5
+  --> $DIR/coercions_same_crate.rs:31:5
    |
 LL | fn cannot_coerce_empty_enum_to_anything(x: UninhabitedEnum) -> A {
    |                                                                - expected `A` because of return type
@@ -10,7 +10,7 @@ LL |     x
               found type `UninhabitedEnum`
 
 error[E0308]: mismatched types
-  --> $DIR/uninhabited_same_crate.rs:35:5
+  --> $DIR/coercions_same_crate.rs:35:5
    |
 LL | fn cannot_coerce_empty_tuple_struct_to_anything(x: UninhabitedTupleStruct) -> A {
    |                                                                               - expected `A` because of return type
@@ -21,7 +21,7 @@ LL |     x
               found type `UninhabitedTupleStruct`
 
 error[E0308]: mismatched types
-  --> $DIR/uninhabited_same_crate.rs:39:5
+  --> $DIR/coercions_same_crate.rs:39:5
    |
 LL | fn cannot_coerce_empty_struct_to_anything(x: UninhabitedStruct) -> A {
    |                                                                    - expected `A` because of return type
@@ -32,7 +32,7 @@ LL |     x
               found type `UninhabitedStruct`
 
 error[E0308]: mismatched types
-  --> $DIR/uninhabited_same_crate.rs:43:5
+  --> $DIR/coercions_same_crate.rs:43:5
    |
 LL | fn cannot_coerce_enum_with_empty_variants_to_anything(x: UninhabitedVariants) -> A {
    |                                                                                  - expected `A` because of return type

src/test/ui/rfc-2008-non-exhaustive/uninhabited/match.stderr

@@ -1,5 +1,5 @@
 error[E0004]: non-exhaustive patterns: type `uninhabited::UninhabitedEnum` is non-empty
-  --> $DIR/uninhabited_match.rs:16:11
+  --> $DIR/match.rs:16:11
    |
 LL |     match x {}
    |           ^

src/test/ui/rfc-2008-non-exhaustive/uninhabited/match_with_exhaustive_patterns.stderr

@@ -1,5 +1,5 @@
 error[E0004]: non-exhaustive patterns: type `uninhabited::UninhabitedEnum` is non-empty
-  --> $DIR/uninhabited_match_with_exhaustive_patterns.rs:19:11
+  --> $DIR/match_with_exhaustive_patterns.rs:19:11
    |
 LL |     match x {}
    |           ^

src/test/ui/rfc-2008-non-exhaustive/uninhabited/patterns_same_crate.stderr

@@ -1,35 +1,35 @@
 error: unreachable pattern
-  --> $DIR/uninhabited_patterns_same_crate.rs:53:9
+  --> $DIR/patterns_same_crate.rs:53:9
    |
 LL |         Some(_x) => (),
    |         ^^^^^^^^
    |
 note: lint level defined here
-  --> $DIR/uninhabited_patterns_same_crate.rs:1:9
+  --> $DIR/patterns_same_crate.rs:1:9
    |
 LL | #![deny(unreachable_patterns)]
    |         ^^^^^^^^^^^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/uninhabited_patterns_same_crate.rs:58:9
+  --> $DIR/patterns_same_crate.rs:58:9
    |
 LL |         Some(_x) => (),
    |         ^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/uninhabited_patterns_same_crate.rs:62:15
+  --> $DIR/patterns_same_crate.rs:62:15
    |
 LL |     while let PartiallyInhabitedVariants::Struct { x } = partially_inhabited_variant() {
    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/uninhabited_patterns_same_crate.rs:66:15
+  --> $DIR/patterns_same_crate.rs:66:15
    |
 LL |     while let Some(_x) = uninhabited_struct() {
    |               ^^^^^^^^
 
 error: unreachable pattern
-  --> $DIR/uninhabited_patterns_same_crate.rs:69:15
+  --> $DIR/patterns_same_crate.rs:69:15
    |
 LL |     while let Some(_x) = uninhabited_tuple_struct() {
    |               ^^^^^^^^