Split out -Wconversion warnings about constant precision into their

Split out -Wconversion warnings about constant precision into their
own subcategory, -Wconstant-conversion, which is on by default.

Tweak the constant folder to give better results in the invalid
case of a negative shift amount.

Implements rdar://problem/6792488

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@118636 91177308-0d34-0410-b5e6-96231b3b80d8