[opt] Add a -strip-module-flags option

The -strip-module-flags option strips llvm.module.flags metadata from a
module at the beginning of the opt pipeline.

This will be used to test whether the output of a pass is debug info
(DI) invariant.

E.g, after applying synthetic debug info to a test case, we'd like to
strip out all DI-related metadata and check that the final IR is
identical to a baseline file without any DI applied, to check that
optimizations aren't inhibited by debug info.

llvm-svn: 333860