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/NetBSD.cpp b/clang/lib/Driver/ToolChains/NetBSD.cpp
index e89ee37..acc5627 100644
--- a/clang/lib/Driver/ToolChains/NetBSD.cpp
+++ b/clang/lib/Driver/ToolChains/NetBSD.cpp
@@ -278,7 +278,8 @@
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
     addOpenMPRuntime(CmdArgs, getToolChain(), Args);
     if (D.CCCIsCXX()) {
-      getToolChain().AddCXXStdlibLibArgs(Args, CmdArgs);
+      if (getToolChain().ShouldLinkCXXStdlib(Args))
+        getToolChain().AddCXXStdlibLibArgs(Args, CmdArgs);
       CmdArgs.push_back("-lm");
     }
     if (NeedsSanitizerDeps)