Fix -Wnull-conversion for long macros.

Move the function to get a macro name from DiagnosticRenderer.cpp to Lexer.cpp
so that other files can use it.  Lexer now has two functions to get the
immediate macro name, the newly added one is better for diagnostic purposes.
Make -Wnull-conversion use this function for better NULL macro detection.

llvm-svn: 258778