[RFC] Clang diagnostic for demangling failures

Code Generation
,
8

Some updated numbers on lit tests with the draft diagnostic enabled:

Failed Tests (5):
  Clang :: CodeGenCXX/PR28523.cpp
  Clang :: CodeGenCXX/mangle-fail.cpp
  Clang :: CodeGenOpenCLCXX/addrspace-of-this.clcpp 
  Clang :: SemaCXX/cxx1y-generic-lambdas-capturing.cpp 
  Clang :: SemaCXX/cxx1y-generic-lambdas.cpp

These failed tests amount to about 25 demangling failures.