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/NaCl.cpp b/clang/lib/Driver/ToolChains/NaCl.cpp
index 5eb5c74..128478d6 100644
--- a/clang/lib/Driver/ToolChains/NaCl.cpp
+++ b/clang/lib/Driver/ToolChains/NaCl.cpp
@@ -133,13 +133,15 @@
 
   if (D.CCCIsCXX() &&
       !Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
-    bool OnlyLibstdcxxStatic =
-        Args.hasArg(options::OPT_static_libstdcxx) && !IsStatic;
-    if (OnlyLibstdcxxStatic)
-      CmdArgs.push_back("-Bstatic");
-    ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
-    if (OnlyLibstdcxxStatic)
-      CmdArgs.push_back("-Bdynamic");
+    if (ToolChain.ShouldLinkCXXStdlib(Args)) {
+      bool OnlyLibstdcxxStatic =
+          Args.hasArg(options::OPT_static_libstdcxx) && !IsStatic;
+      if (OnlyLibstdcxxStatic)
+        CmdArgs.push_back("-Bstatic");
+      ToolChain.AddCXXStdlibLibArgs(Args, CmdArgs);
+      if (OnlyLibstdcxxStatic)
+        CmdArgs.push_back("-Bdynamic");
+    }
     CmdArgs.push_back("-lm");
   }