Update description for CLANG_DEFAULT_CXX_STDLIB and add comment. NFC

We agreed to call it "platform default" instead of "architecture default".
(see D17286)

llvm-svn: 276618
diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index b4491d6..66b76cd 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -577,6 +577,8 @@
         << A->getAsString(Args);
   }
 
+  // If no argument was provided or its value was invalid, look for the
+  // default unless forced or configured to take the platform default.
   if (!HasValidType && (ForcePlatformDefault ||
       !ParseCXXStdlibType(CLANG_DEFAULT_CXX_STDLIB, Type)))
     Type = GetDefaultCXXStdlibType();