[frontend] Don't include the C++ stdlib for -x assembler-with-cpp

The new C++ stdlib warning added in r335081 gets
triggered when compiling an assembly file with -x assembler-with-cpp.
This commit ensures that the C++ stdlib is not included when compiling assembly.
In general, it's not really useful to include the C++ stdlib search path when
compiling assembly source.

rdar://41359632

Differential Revision: https://reviews.llvm.org/D48736

llvm-svn: 335940
diff --git a/clang/lib/Frontend/InitHeaderSearch.cpp b/clang/lib/Frontend/InitHeaderSearch.cpp
index 85aeec4..8a70404 100644
--- a/clang/lib/Frontend/InitHeaderSearch.cpp
+++ b/clang/lib/Frontend/InitHeaderSearch.cpp
@@ -470,8 +470,8 @@
     break;
   }
 
-  if (Lang.CPlusPlus && HSOpts.UseStandardCXXIncludes &&
-      HSOpts.UseStandardSystemIncludes) {
+  if (Lang.CPlusPlus && !Lang.AsmPreprocessor &&
+      HSOpts.UseStandardCXXIncludes && HSOpts.UseStandardSystemIncludes) {
     if (HSOpts.UseLibcxx) {
       if (triple.isOSDarwin()) {
         // On Darwin, libc++ may be installed alongside the compiler in