ASTMatchers: Bound node results are always const, make selectFirst's template argument implicitly const.

This avoids adding const to every user of selectFirst and also allows it to
match TypeLocs which BoundNodes doesn't use magic const removal specializations
for. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213737 91177308-0d34-0410-b5e6-96231b3b80d8