[modules] Don't leak -M flags for dependency file generation into the module

build process when we implicitly build a module. Previously, we'd create the
specified .d file once for each implicitly-built module and then finally
overwrite it with the correct contents after the requested build completes.
(This fails if you use stdout as a dependency file, which is what the provided
testcase does, and is how I discovered this brokenness.)

llvm-svn: 244412