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/MinGW.cpp b/clang/lib/Driver/ToolChains/MinGW.cpp
index 7550bab..1c5eeb3 100644
--- a/clang/lib/Driver/ToolChains/MinGW.cpp
+++ b/clang/lib/Driver/ToolChains/MinGW.cpp
@@ -185,8 +185,7 @@
 
   // TODO: Add profile stuff here
 
-  if (D.CCCIsCXX() &&
-      !Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
+  if (TC.ShouldLinkCXXStdlib(Args)) {
     bool OnlyLibstdcxxStatic = Args.hasArg(options::OPT_static_libstdcxx) &&
                                !Args.hasArg(options::OPT_static);
     if (OnlyLibstdcxxStatic)