Add DebugIR pass -- emits IR file and replace source lines with IR lines in MD

- requires existing debug information to be present
- fixes up file name and line number information in metadata
- emits a "<orig_filename>-debug.ll" succinct IR file (without !dbg metadata
  or debug intrinsics) that can be read by a debugger
- initialize pass in opt tool to enable the "-debug-ir" flag
- lit tests to follow



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181467 91177308-0d34-0410-b5e6-96231b3b80d8

tools/opt/opt.cpp

@@ -567,6 +567,7 @@ int main(int argc, char **argv) {
   // Initialize passes
   PassRegistry &Registry = *PassRegistry::getPassRegistry();
   initializeCore(Registry);
+  initializeDebugIRPass(Registry);
   initializeScalarOpts(Registry);
   initializeObjCARCOpts(Registry);
   initializeVectorization(Registry);