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

[mstorsjo]

Summary

The arguments passed in this option were passed onto the child
process, but we still blindly 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).

[aganea]

Something was missing in the description:

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.

Looks good otherwise!

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

struct RcOptions {
   bool IsDryRun = false;
   // Set the default language; choose en-US arbitrarily.
   unsigned LangId = (/*PrimaryLangId*/ 0x09) | (/*SubLangId*/ 0x01 << 10);
 };
 
 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);
     if (ClangOrErr) {
       Clang = *ClangOrErr;
     } else {
       errs() << "llvm-rc: Unable to find clang, skipping preprocessing."
              << "\n";
for (const auto &A : Args) {
       sys::printArg(outs(), A, Opts.PrintCmdAndExit);
     }
     outs() << "\n";
     if (Opts.PrintCmdAndExit)
       exit(0);
   }
   // 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.");
   }
   return true;
 }
 
 static std::pair<bool, std::string> isWindres(llvm::StringRef Argv0) {
   StringRef ProgName = llvm::sys::path::stem(Argv0);