Add test for --extern alloc=librustc.rlib

Author: ehuss

src/test/run-make-fulldeps/extern-flag-fun/Makefile

@@ -14,3 +14,6 @@ all:
 		--extern bar=$(TMPDIR)/libbar.rlib \
 		--extern bar=$(TMPDIR)/libbar-a.rlib
 	$(RUSTC) foo.rs --extern bar=$(TMPDIR)/libbar.rlib
+	# Try to be sneaky and load a private crate from with a non-private name.
+	$(RUSTC) rustc.rs -Zforce-unstable-if-unmarked --crate-type=rlib
+	$(RUSTC) gated_unstable.rs --extern alloc=$(TMPDIR)/librustc.rlib 2>&1 | $(CGREP) 'rustc_private'

src/test/run-make-fulldeps/extern-flag-fun/gated_unstable.rs

@@ -0,0 +1,3 @@
+extern crate alloc;
+
+fn main() {}

src/test/run-make-fulldeps/extern-flag-fun/rustc.rs

@@ -0,0 +1 @@
+pub fn foo() {}