Patch to warn if 'override' is missing

Patch to warn if 'override' is missing
for an overriding method if class has at least one
'override' specified on one of its methods.
Reviewed by Doug Gregor. rdar://18295240
(I have already checked in all llvm files with missing 'override'
 methods and Bob Wilson has fixed a TableGen of FastISel so
 no warnings are expected from build of llvm after this patch.
 I have already verified this).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218925 91177308-0d34-0410-b5e6-96231b3b80d8