Refactor stripDebugInfo(Function) to handle intrinsic

This moves the code that handles stripping debug info intrinsic from
 StripDebugInfo(Module) to StripDebugInfo(Function). The latter is
already walking every instructions so it makes sense to do it at the
same time.
This makes also stripDebugInfo(Function) as an API more useful: it
is really dropping every debug info in the Function.
Finally the existing code is trigerring an assertion when the Module
is not fully materialized.

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 268847