[analyzer] Add a utility method that allows to find the macro name used
at the given location. 

This could be useful when checkers' logic depends on whether a function
is called with a given macro argument.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@148516 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed