Generalize bugpoint's concept of a "safe" backend, and add options

to allow the "safe" backend to be run with a different path, and/or
with different command-line options.

This enables the following use cases:
 - bugpoint llc against an llc command from a different build
 - bugpoint llc against the same llc with different command-line options
 - and more...

Also, document the existing "custom" interpreter options.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60681 91177308-0d34-0410-b5e6-96231b3b80d8

tools/bugpoint/BugDriver.cpp

@@ -65,7 +65,8 @@ std::string llvm::getPassesString(const std::vector<const PassInfo*> &Passes) {
 BugDriver::BugDriver(const char *toolname, bool as_child, bool find_bugs,
                      unsigned timeout, unsigned memlimit)
   : ToolName(toolname), ReferenceOutputFile(OutputFile),
-    Program(0), Interpreter(0), cbe(0), gcc(0), run_as_child(as_child),
+    Program(0), Interpreter(0), SafeInterpreter(0), gcc(0),
+    run_as_child(as_child),
     run_find_bugs(find_bugs), Timeout(timeout), MemoryLimit(memlimit) {}