Remove cache for macro arg stringization

Summary:
The cache recorded the wrong expansion location for all but the first
stringization. It seems uncommon to stringize the same macro argument
multiple times, so this cache doesn't seem that important.

Fixes PR39942

Reviewers: vsk, rsmith

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D65428

llvm-svn: 367337