Much like we silence warnings about -flto in many cases to facilitate
simplicity in build systems, silence '-stdlib=libc++' when linking. Even
if we're not linking C++ code per-se, we may be passing this flag so
that when we are linking C++ code we pick up the desired standard
library. While most build systems already provide separate C and C++
compile flags, many conflate link flags. Sadly, CMake is among them
causing this warning in a libc++ selfhost.

llvm-svn: 231559
diff --git a/clang/lib/Driver/Tools.cpp b/clang/lib/Driver/Tools.cpp
index a80214e..3c6827a 100644
--- a/clang/lib/Driver/Tools.cpp
+++ b/clang/lib/Driver/Tools.cpp
@@ -7704,6 +7704,8 @@
       CmdArgs.push_back("-Bdynamic");
     CmdArgs.push_back("-lm");
   }
+  // Silence warnings when linking C code with a C++ '-stdlib' argument.
+  Args.ClaimAllArgs(options::OPT_stdlib_EQ);
 
   if (!Args.hasArg(options::OPT_nostdlib)) {
     if (!Args.hasArg(options::OPT_nodefaultlibs)) {