Introduce the lambda scope before determining explicit captures, which

cleans up and improves a few things:
  - We get rid of the ugly dance of computing all of the captures in
  data structures that clone those of CapturingScopeInfo, centralizing
  the logic for accessing/updating these data structures
  - We re-use the existing capture logic for 'this', which actually
  works now.

Cleaned up some diagnostic wording in minor ways as well.

llvm-svn: 149516