[Sema] Avoid -Wshadow warnings for shadowed variables that

aren't captured by lambdas with a default capture specifier

This commit is a follow-up to r286354. It avoids the -Wshadow warning for
variables which shadow variables that aren't captured by lambdas with a default
capture specifier. It provides an additional note that points to location of
the capture.

The old behaviour is preserved with -Wshadow-all or -Wshadow-uncaptured-local.

rdar://14984176

Differential Revision: https://reviews.llvm.org/D26448

llvm-svn: 286465