[Bugpoint] Allow fallback to clang

[davide]

Summary

FreeBSD doesn't ship with gcc by default anymore, so some bugpoint tests fail on a stock installation.
In D12273 I proposed to XFAIL the tests on FreeBSD, but Hal replied proposing to fallback on clang if gcc is not available.
This patch implements the proposed logic. Hal, I listed you as reviewer because you're the one that came up with this idea and apparently Bugpoint is currently unowned. Feel free to remove yourself and/or suggest somebody else that should review this.

[• rafael]

Changing GCCBinary in here looks odd. It would be particularly odd if the --gcc option was given.

How about changing the default of the option to "" and in main setting it to "gcc" or "clang" if the user didn't pass -gcc?

[davide]

I think your idea is much better, thanks.

tools/bugpoint/ExecutionDriver.cpp

ToolArgv("tool-args", cl::Positional, cl::desc("<tool arguments>..."),
            cl::ZeroOrMore, cl::PositionalEatsArgs);
 
   cl::list<std::string>
   SafeToolArgv("safe-tool-args", cl::Positional,
                cl::desc("<safe-tool arguments>..."),
                cl::ZeroOrMore, cl::PositionalEatsArgs);
 
   cl::opt<std::string>
-  GCCBinary("gcc", cl::init("gcc"),
+  GCCBinary("gcc", cl::init(""),
               cl::desc("The gcc binary to use. (default 'gcc')"));
 
   cl::list<std::string>
   GCCToolArgv("gcc-tool-args", cl::Positional,
               cl::desc("<gcc-tool arguments>..."),
               cl::ZeroOrMore, cl::PositionalEatsArgs);
 }
 
 //===----------------------------------------------------------------------===//
 // BugDriver method implementation
 //
 
 /// initializeExecutionEnvironment - This method is used to set up the
 /// environment for executing LLVM programs.
 ///
 bool BugDriver::initializeExecutionEnvironment() {
   outs() << "Initializing execution environment: ";
 
   // Create an instance of the AbstractInterpreter interface as specified on
   // the command line
   SafeInterpreter = nullptr;
   std::string Message;
 
+  // Try to fall-back to clang if the user didn't pass GCC as option.
+  if (GCCBinary == "")
+    GCCBinary = "clang";
+
   switch (InterpreterSel) {
   case AutoPick:
     if (!Interpreter) {
       InterpreterSel = RunJIT;
       Interpreter = AbstractInterpreter::createJIT(getToolName(), Message,
                                                    &ToolArgv);
     }
     if (!Interpreter) {