set up the machinery for a MacroArgs cache hanging off Preprocessor.
We creating and free thousands of MacroArgs objects (and the related
std::vectors hanging off them) for the testcase in PR5610 even though
there are only ~20 live at a time.  This doesn't actually use the 
cache yet.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@91391 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed