[Sema] Avoid -Wshadow warnings for shadowed variables that aren't captured

by lambdas with an explicit capture list

This commit avoids the -Wshadow warning for variables which shadow variables
that aren't captured by lambdas with an explicit capture list. It provides an
additional note that points to location of the explicit capture.

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

rdar://17135966

Differential Revision: https://reviews.llvm.org/D26278

llvm-svn: 286354