Improve checking of explicit captures in a C++11 lambda expression:

  - Actually building the var -> capture mapping properly (there was an off-by-one error)
  - Keeping track of the source location of each capture
  - Minor QoI improvements, e.g, highlighing the prior capture if
  there are multiple captures, pointing at the variable declaration we
  found if we reject it.

As part of this, add standard citations for the various semantic
checks we perform, and note where we're not performing those checks as
we should.

llvm-svn: 149462