clang-cl: Better error message when trying to compile stdin (PR18640)

We should suggest using /Tc or /Tp to set the input type, instead of
erroneously suggesting -x, which isn't a clang-cl flag.

llvm-svn: 200362
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index ca3e50f..bd43fbe 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -1030,7 +1030,8 @@
           // Otherwise emit an error but still use a valid type to avoid
           // spurious errors (e.g., no inputs).
           if (!Args.hasArgNoClaim(options::OPT_E) && !CCCIsCPP())
-            Diag(clang::diag::err_drv_unknown_stdin_type);
+            Diag(IsCLMode() ? clang::diag::err_drv_unknown_stdin_type_clang_cl
+                            : clang::diag::err_drv_unknown_stdin_type);
           Ty = types::TY_C;
         } else {
           // Otherwise lookup by extension.