Fix usage of rustc --ls on invalid files

Closes #11259

src/librustc/lib/llvm.rs

@@ -1936,7 +1936,7 @@ pub fn new(llmb: MemoryBufferRef) -> Option<ObjectFile> {
         unsafe {
             let llof = llvm::LLVMCreateObjectFile(llmb);
             if llof as int == 0 {
-                llvm::LLVMDisposeMemoryBuffer(llmb);
+                // LLVMCreateObjectFile took ownership of llmb
                 return None
             }
 

src/test/run-make/ls-metadata/Makefile

+-include ../tools.mk
+
+all:
+	$(RUSTC) foo.rs
+	$(RUSTC) --ls $(TMPDIR)/foo
+	touch $(TMPDIR)/bar
+	$(RUSTC) --ls $(TMPDIR)/bar

src/test/run-make/ls-metadata/foo.rs

+fn main() {}