[Driver] Support file paths with -ccc-gcc-name

[al3xtjames]

Summary

When a file path is specified for gcc-name, don't prepend the target
triple. This allows -target and -ccc-gcc-name to be used together when
a file path is specified.

clang/lib/Driver/ToolChains/Gnu.cpp

void tools::gcc::Common::ConstructJob(Compilation &C, const JobAction &JA,
   const char *GCCName;
   if (!customGCCName.empty())
     GCCName = customGCCName.c_str();
   else if (D.CCCIsCXX()) {
     GCCName = "g++";
   } else
     GCCName = "gcc";
 
-  const char *Exec = Args.MakeArgString(getToolChain().GetProgramPath(GCCName));
+  const char *Exec;
+  if (llvm::sys::fs::exists(GCCName))
+    Exec = GCCName;
+  else
+    Exec = Args.MakeArgString(getToolChain().GetProgramPath(GCCName));
+
   C.addCommand(std::make_unique<Command>(JA, *this,
                                          ResponseFileSupport::AtFileCurCP(),
                                          Exec, CmdArgs, Inputs, Output));
 }
 
 void tools::gcc::Preprocessor::RenderExtraToolArgs(
     const JobAction &JA, ArgStringList &CmdArgs) const {
   CmdArgs.push_back("-E");