Introduce -nostdlib++ flag to disable linking the C++ standard library.

Projects that want to statically link their own C++ standard library currently
need to pass -nostdlib or -nodefaultlibs, which also disables linking of the
builtins library, -lm, and so on. Alternatively, they could use `clang` instead
of `clang++`, but that already disables implicit addition of -lm on some
toolchains.

Add a dedicated flag -nostdlib++ that disables just linking of libc++ /
libstdc++. This is analogous to -nostdinc++.

https://reviews.llvm.org/D35780

llvm-svn: 308997
diff --git a/clang/lib/Driver/ToolChains/Darwin.cpp b/clang/lib/Driver/ToolChains/Darwin.cpp
index 2742870..bd8516c 100644
--- a/clang/lib/Driver/ToolChains/Darwin.cpp
+++ b/clang/lib/Driver/ToolChains/Darwin.cpp
@@ -549,10 +549,9 @@
         Args.MakeArgString(Twine("-threads=") + llvm::to_string(Parallelism)));
   }
 
+  if (getToolChain().ShouldLinkCXXStdlib(Args))
+    getToolChain().AddCXXStdlibLibArgs(Args, CmdArgs);
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
-    if (getToolChain().getDriver().CCCIsCXX())
-      getToolChain().AddCXXStdlibLibArgs(Args, CmdArgs);
-
     // link_ssp spec is empty.
 
     // Let the tool chain choose which runtime library to link.