[CMake] Add option to switch default C++ stdlib

With this option one can optionally override the architecture dependent
default library to use if no -stdlib= is provided on compiler invocation.

Differential Revision: http://reviews.llvm.org/D15920

llvm-svn: 260662
diff --git a/clang/lib/Driver/ToolChains.h b/clang/lib/Driver/ToolChains.h
index d9c87b9..127f37f 100644
--- a/clang/lib/Driver/ToolChains.h
+++ b/clang/lib/Driver/ToolChains.h
@@ -512,6 +512,7 @@
   TranslateArgs(const llvm::opt::DerivedArgList &Args,
                 const char *BoundArch) const override;
 
+  CXXStdlibType GetDefaultCXXStdlibType() const override;
   ObjCRuntime getDefaultObjCRuntime(bool isNonFragile) const override;
   bool hasBlocksRuntime() const override;
 
@@ -699,7 +700,7 @@
   bool IsMathErrnoDefault() const override { return false; }
   bool IsObjCNonFragileABIDefault() const override { return true; }
 
-  CXXStdlibType GetCXXStdlibType(const llvm::opt::ArgList &Args) const override;
+  CXXStdlibType GetDefaultCXXStdlibType() const override;
   void AddClangCXXStdlibIncludeArgs(
       const llvm::opt::ArgList &DriverArgs,
       llvm::opt::ArgStringList &CC1Args) const override;