Fix use of invalidated iterator bug in AST match finder.

Pulled out the cache clearing in the case of descendant matching, too,
for consistency, also it is not technically needed there.

FIXME: Make cache size configurable and add unit test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185820 91177308-0d34-0410-b5e6-96231b3b80d8