Give the RecursiveASTVisitor a configuration function

Give the RecursiveASTVisitor a configuration function
shouldWalkTypesOfTypeLocs() that determines whether it should walk the
Types within TypeLocs. This walk is redundant, but perhaps required
for some clients. Disabling this redundant walk in the unexpanded
parameter pack finder produces better results, because we get
parameter packs with source location info *unless* such source
location information isn't available.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122287 91177308-0d34-0410-b5e6-96231b3b80d8