Fix for bug 38508 - Don't do PCH processing when only generating preprocessor output

This clang-cl driver change removes the PCH options when we are only generating
preprocessed output. This is similar to the behavior of Y-.

Patch by: mikerice
Differential Revision: https://reviews.llvm.org/D50640

llvm-svn: 340025
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 350ee0e3..6527c19 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -3010,9 +3010,10 @@
     Args.eraseArg(options::OPT__SLASH_Yc);
     YcArg = nullptr;
   }
-  if (Args.hasArg(options::OPT__SLASH_Y_)) {
-    // /Y- disables all pch handling.  Rather than check for it everywhere,
-    // just remove clang-cl pch-related flags here.
+  if (FinalPhase == phases::Preprocess || Args.hasArg(options::OPT__SLASH_Y_)) {
+    // If only preprocessing or /Y- is used, all pch handling is disabled.
+    // Rather than check for it everywhere, just remove clang-cl pch-related
+    // flags here.
     Args.eraseArg(options::OPT__SLASH_Fp);
     Args.eraseArg(options::OPT__SLASH_Yc);
     Args.eraseArg(options::OPT__SLASH_Yu);