Add support for optimization reports.

Summary:
This patch adds a new flag -Rpass=. The flag indicates the name
of the optimization pass that should emit remarks stating when it
made a transformation to the code.

This implements the design I proposed in:

https://docs.google.com/document/d/1FYUatSjZZO-zmFBxjOiuOzAy9mhHA8hqdvklZv68WuQ/edit?usp=sharing

Other changes:
- Add DiagnosticIDs::isRemark(). Use it in printDiagnosticOptions to
  print "-R" instead of "-W" in the diagnostic message.

- In BackendConsumer::OptimizationRemarkHandler, get a SourceLocation
  object out of the file name, line and column number. Use that location
  in the call to Diags.Report().

- When -Rpass is used without debug info a note is emitted alerting
  the user that they need to use -gline-tables-only -gcolumn-info to
  get this information.

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D3226

llvm-svn: 206401
diff --git a/clang/lib/Frontend/TextDiagnosticPrinter.cpp b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
index 994a8f7..3b5eca6 100644
--- a/clang/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/clang/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -81,7 +81,8 @@
 
     StringRef Opt = DiagnosticIDs::getWarningOptionForDiag(Info.getID());
     if (!Opt.empty()) {
-      OS << (Started ? "," : " [") << "-W" << Opt;
+      OS << (Started ? "," : " [")
+         << (DiagnosticIDs::isRemark(Info.getID()) ? "-R" : "-W") << Opt;
       Started = true;
     }
   }