Move Sema::RequireCompleteType() and Sema::RequireCompleteExprType()

Move Sema::RequireCompleteType() and Sema::RequireCompleteExprType()
off PartialDiagnostic. PartialDiagnostic is rather heavyweight for
something that is in the critical path and is rarely used. So, switch
over to an abstract-class-based callback mechanism that delays most of
the work until a diagnostic is actually produced. Good for ~11k code
size reduction in the compiler and 1% speedup in -fsyntax-only on the
code in <rdar://problem/11004361>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156176 91177308-0d34-0410-b5e6-96231b3b80d8