Change our basic strategy for avoiding deprecation warnings when the decl use

appears in a deprecated context.  In the new strategy, we emit the warnings
as usual unless we're currently parsing a declaration, where "declaration" is
restricted to mean a decl group or a few special cases in Objective C.  If
we *are* parsing a declaration, we queue up the deprecation warnings until
the declaration has been completely parsed, and then emit them only if the
decl is not deprecated.
We also standardize the bookkeeping for deprecation so as to avoid special cases.

llvm-svn: 85998