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). 

llvm-svn: 218925