add support for LLVM pass optimization remarks

[thestinger]

http://clang.llvm.org/docs/UsersManual.html#options-to-emit-optimization-reports

[kmcallister]

Looking into this.

[sanxiyn]

FYI, this is done using LLVMContext::setDiagnosticHandler. Look at Clang's CodeGenAction.cpp for an example handler implementation.