Remove unused Prev pointer from MacroInfo chain.

Remove pointless MICache: it only ever contained up to 1 object, and was only
non-empty when recovering from an error. There's no performance or memory win
from maintaining this cache.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213825 91177308-0d34-0410-b5e6-96231b3b80d8