Return a reference instead of vector copy for parentmap queries.

The map is immutable until the whole ASTContext dies. While there
movify a couple of copies in ASTMatchFinder away. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219062 91177308-0d34-0410-b5e6-96231b3b80d8