Add profiling support, through the rustc -Z profile flag.

When -Z profile is passed, the GCDAProfiling LLVM pass is added
to the pipeline, which uses debug information to instrument the IR.
After compiling with -Z profile, the $(OUT_DIR)/$(CRATE_NAME).gcno
file is created, containing initial profiling information.
After running the program built, the $(OUT_DIR)/$(CRATE_NAME).gcda
file is created, containing branch counters.

The created *.gcno and *.gcda files can be processed using
the "llvm-cov gcov" and "lcov" tools. The profiling data LLVM
generates does not faithfully follow the GCC's format for *.gcno
and *.gcda files, and so it will probably not work with other tools
(such as gcov itself) that consume these files.

src/rustllvm/RustWrapper.cpp

@@ -466,6 +466,10 @@ extern "C" void LLVMRustAddModuleFlag(LLVMModuleRef M, const char *Name,
   unwrap(M)->addModuleFlag(Module::Warning, Name, Value);
 }
 
+extern "C" void LLVMRustMetadataAsValue(LLVMContextRef C, LLVMRustMetadataRef MD) {
+  wrap(MetadataAsValue::get(*unwrap(C), unwrap(MD)));
+}
+
 extern "C" LLVMRustDIBuilderRef LLVMRustDIBuilderCreate(LLVMModuleRef M) {
   return new DIBuilder(*unwrap(M));
 }