Cache macro expander objects to avoid thrashing malloc in heavy expansion situations.

This doesn't significantly improve carbon.h, but it does speed up
INPUTS/macro_pounder_obj.c by 48%



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@39864 91177308-0d34-0410-b5e6-96231b3b80d8