[llvm-rc] Respect the executable specified in the --preprocessor command

The arguments passed in this option were passed onto the child
process, but we still blindly used the clang binary that we had
found to sys::ExecuteAndWait as the intended executable to run.

If the user hasn't specified any custom --preprocessor command,
Args[0] is equal to the variable Clang.

This doesn't affect any tests, since the tests only print the
arguments it would try to execute (but not the first parameter to
sys::ExecuteAndWait), but there's no testes for executing it
(and validating that it did execute the right thing).

Differential Revision: https://reviews.llvm.org/D146793

(cherry picked from commit d2fa6b6)

Author: mstorsjo

llvm/tools/llvm-rc/llvm-rc.cpp

@@ -226,7 +226,7 @@ struct RcOptions {
 bool preprocess(StringRef Src, StringRef Dst, const RcOptions &Opts,
                 const char *Argv0) {
   std::string Clang;
-  if (Opts.PrintCmdAndExit) {
+  if (Opts.PrintCmdAndExit || !Opts.PreprocessCmd.empty()) {
     Clang = "clang";
   } else {
     ErrorOr<std::string> ClangOrErr = findClang(Argv0, Opts.Triple);
@@ -266,7 +266,7 @@ bool preprocess(StringRef Src, StringRef Dst, const RcOptions &Opts,
   }
   // The llvm Support classes don't handle reading from stdout of a child
   // process; otherwise we could avoid using a temp file.
-  int Res = sys::ExecuteAndWait(Clang, Args);
+  int Res = sys::ExecuteAndWait(Args[0], Args);
   if (Res) {
     fatalError("llvm-rc: Preprocessing failed.");
   }