clang-cl pch test: Instead of copying the input, use /Fp to not write into the test directory.

Also fix a bug with /Fp and absolute paths uncovered by this.
Follow-up to r262487.

llvm-svn: 262541
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 8965d3d..f8d6fc7 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -2246,7 +2246,7 @@
   }
 
   // As an annoying special case, PCH generation doesn't strip the pathname.
-  if (JA.getType() == types::TY_PCH) {
+  if (JA.getType() == types::TY_PCH && !IsCLMode()) {
     llvm::sys::path::remove_filename(BasePath);
     if (BasePath.empty())
       BasePath = NamedOutput;