Add a run-make test for command aliases

src/test/run-make/override-aliased-flags/Makefile

@@ -0,0 +1,22 @@
+-include ../../run-make-fulldeps/tools.mk
+
+# FIXME: it would be good to check that it's actually the rightmost flags
+# that are used when multiple flags are specified, but I can't think of a
+# reliable way to check this.
+
+all:
+	# Test that `-O` and `-C opt-level` can be specified multiple times.
+	# The rightmost flag will be used over any previous flags.
+	$(RUSTC) -O -O main.rs
+	$(RUSTC) -O -C opt-level=0 main.rs
+	$(RUSTC) -C opt-level=0 -O main.rs
+	$(RUSTC) -C opt-level=0 -C opt-level=2 main.rs
+	$(RUSTC) -C opt-level=2 -C opt-level=0 main.rs
+
+	# Test that `-g` and `-C debuginfo` can be specified multiple times.
+	# The rightmost flag will be used over any previous flags.
+	$(RUSTC) -g -g main.rs
+	$(RUSTC) -g -C debuginfo=0 main.rs
+	$(RUSTC) -C debuginfo=0 -g main.rs
+	$(RUSTC) -C debuginfo=0 -C debuginfo=2 main.rs
+	$(RUSTC) -C debuginfo=2 -C debuginfo=0 main.rs

src/test/run-make/override-aliased-flags/main.rs

@@ -0,0 +1 @@
+fn main() {}