Avoid -Wshadow warnings about constructor parameters named after fields

Usually these parameters are used solely to initialize the field in the
initializer list, and there is no real shadowing confusion.

There is a new warning under -Wshadow called
-Wshadow-field-in-constructor-modified. It attempts to find
modifications of such constructor parameters that probably intended to
modify the field.

It has some false negatives, though, so there is another warning group,
-Wshadow-field-in-constructor, which always warns on this special case.
For users who just want the old behavior and don't care about these fine
grained groups, we have a new warning group called -Wshadow-all that
activates everything.

Fixes PR16088.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18271

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267957 91177308-0d34-0410-b5e6-96231b3b80d8