Improve diagnostics for invalid use of non-static members / this:

* s/nonstatic/non-static/ in the diagnostics, since the latter form outvoted
  the former by 28-2 in our diagnostics.
* Fix the "use of member in static member function" diagnostic to correctly
  detect this situation inside a block or lambda.
* Produce a more specific "invalid use of non-static member" diagnostic for
  the case where a nested class member refers to a member of a
  lexically-surrounding class.

llvm-svn: 154073