Fix condition for "missing struct" diagnostic on tuple structs

The check previously matched this, and suggested adding a missing
`struct`:

pub Foo(...):

It was probably intended to match this instead (semicolon instead of
colon):

pub Foo(...);

compiler/rustc_parse/src/parser/item.rs

@@ -412,7 +412,7 @@ impl<'a> Parser<'a> {
             } else if self.check(&token::OpenDelim(Delimiter::Brace)) {
                 self.bump(); // `{`
                 ("fn", kw_name, false)
-            } else if self.check(&token::Colon) {
+            } else if self.check(&token::Semi) {
                 let kw = "struct";
                 (kw, kw, false)
             } else {

tests/ui/pub/pub-ident-fn-3.rs

@@ -2,7 +2,7 @@
 
 mod foo {
     pub bar();
-    //~^ ERROR missing `fn` or `struct` for function or struct definition
+    //~^ ERROR missing `struct` for struct definition
 }
 
 fn main() {}

tests/ui/pub/pub-ident-fn-3.stderr

@@ -1,8 +1,13 @@
-error: missing `fn` or `struct` for function or struct definition
+error: missing `struct` for struct definition
   --> $DIR/pub-ident-fn-3.rs:4:8
    |
 LL |     pub bar();
-   |     ---^--- help: if you meant to call a macro, try: `bar!`
+   |        ^
+   |
+help: add `struct` here to parse `bar` as a public struct
+   |
+LL |     pub struct bar();
+   |         ++++++
 
 error: aborting due to previous error
 

tests/ui/pub/pub-ident-fn-or-struct-2.rs

@@ -1,4 +1,4 @@
 pub S();
-//~^ ERROR missing `fn` or `struct` for function or struct definition
+//~^ ERROR missing `struct` for struct definition
 
 fn main() {}

tests/ui/pub/pub-ident-fn-or-struct-2.stderr

@@ -1,8 +1,13 @@
-error: missing `fn` or `struct` for function or struct definition
+error: missing `struct` for struct definition
   --> $DIR/pub-ident-fn-or-struct-2.rs:1:4
    |
 LL | pub S();
-   | ---^- help: if you meant to call a macro, try: `S!`
+   |    ^
+   |
+help: add `struct` here to parse `S` as a public struct
+   |
+LL | pub struct S();
+   |     ++++++
 
 error: aborting due to previous error
 

tests/ui/pub/pub-ident-struct-4.fixed

@@ -0,0 +1,6 @@
+// run-rustfix
+
+pub struct T(String);
+//~^ ERROR missing `struct` for struct definition
+
+fn main() {}

tests/ui/pub/pub-ident-struct-4.rs

@@ -0,0 +1,6 @@
+// run-rustfix
+
+pub T(String);
+//~^ ERROR missing `struct` for struct definition
+
+fn main() {}

tests/ui/pub/pub-ident-struct-4.stderr

@@ -0,0 +1,13 @@
+error: missing `struct` for struct definition
+  --> $DIR/pub-ident-struct-4.rs:3:4
+   |
+LL | pub T(String);
+   |    ^
+   |
+help: add `struct` here to parse `T` as a public struct
+   |
+LL | pub struct T(String);
+   |     ++++++
+
+error: aborting due to previous error
+