Teach CIndex's cursor visitor to restrict its traversal to a specific

Teach CIndex's cursor visitor to restrict its traversal to a specific
region of interest (if provided). Implement clang_getCursor() in terms
of this traversal rather than using the Index library; the unified
cursor visitor is more complete, and will be The Way Forward.

Minor other tweaks needed to make this work:
  - Extend Preprocessor::getLocForEndOfToken() to accept an offset
  from the end, making it easy to move to the last character in the
  token (rather than just past the end of the token).
  - In Lexer::MeasureTokenLength(), the length of whitespace is zero.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@94200 91177308-0d34-0410-b5e6-96231b3b80d8