Fix skipping of flags in getClangStripDependencyFileAdjuster

Summary:
The ArgumentsAdjuster returned from `getClangStripDependencyFileAdjuster` will
skip dependency flags, and also their associated values for those flags that
take an argument. This change corrects the handling of the `-MD` and `-MMD`
flags, which do not take an argument.

Reviewers: saugustine, klimek, alexshap

Reviewed By: alexshap

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40024

llvm-svn: 318529