[clang-tidy] Fix identifier naming in macro args.

Summary:
clang-tidy should fix identifier naming even when the identifier is
referenced inside a macro expansion, provided that the identifier enters
the macro expansion completely within a macro argument.

For example, this will allow fixes to the naming of the identifier
'global' when it is declared and used as follows:

  int global;
  #define USE_IN_MACRO(m) auto use_##m = m
  USE_IN_MACRO(global);

Reviewers: alexfh

Subscribers: jlebar, cfe-commits

Differential Revision: https://reviews.llvm.org/D25450

llvm-svn: 284992