add support for -x c++-header, update comment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63924 91177308-0d34-0410-b5e6-96231b3b80d8

Driver/clang.cpp

@@ -267,6 +267,7 @@ enum LangKind {
   langkind_c_pch,
   langkind_cxx,
   langkind_cxx_cpp,
+  langkind_cxx_pch,
   langkind_objc,
   langkind_objc_cpp,
   langkind_objc_pch,
@@ -276,9 +277,7 @@ enum LangKind {
 };
 
 /* TODO: GCC also accepts:
-   c-header c++-header objective-c-header objective-c++-header
-   assembler
-   ada, f77*, ratfor (!), f95, java, treelang
+   assembler, ada, f77*, ratfor (!), f95, java, treelang
  */
 static llvm::cl::opt<LangKind>
 BaseLang("x", llvm::cl::desc("Base language to compile"),
@@ -301,6 +300,8 @@ BaseLang("x", llvm::cl::desc("Base language to compile"),
                                "Precompiled C header"),
                     clEnumValN(langkind_objc_pch, "objective-c-header",
                                "Precompiled Objective-C header"),
+                    clEnumValN(langkind_cxx_pch, "c++-header",
+                               "Precompiled C++ header"),
                     clEnumValN(langkind_objcxx_pch, "objective-c++-header",
                                "Precompiled Objective-C++ header"),
                     clEnumValEnd));
@@ -386,6 +387,10 @@ static bool InitializeLangOptions(LangOptions &Options, LangKind LK){
     LK = langkind_objc;
     PCH = true;
     break;
+  case langkind_cxx_pch:
+    LK = langkind_cxx;
+    PCH = true;
+    break;
   case langkind_objcxx_pch:
     LK = langkind_objcxx;
     PCH = true;