Correctly emit certain implicit references to 'self' even within
a lambda.

Bug #1 is that CGF's CurFuncDecl was "stuck" at lambda invocation
functions.  Fix that by generally improving getNonClosureContext
to look through lambdas and captured statements but only report
code contexts, which is generally what's wanted.  Audit uses of
CurFuncDecl and getNonClosureAncestor for correctness.

Bug #2 is that lambdas weren't specially mapping 'self' when inside
an ObjC method.  Fix that by removing the requirement for that
and using the normal EmitDeclRefLValue path in LoadObjCSelf.

rdar://13800041