Change how we distinguish bitfield widths, in-class
initializers, and captured VLA types so that we can
answer questions like "is this a bit-field" without
looking at the enclosing DeclContext.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219522 91177308-0d34-0410-b5e6-96231b3b80d8