File::Clear() -> File::TakeStreamAndClear()
File::Clear() is an ugly function. It's only used in one place,
which is the swig typemaps for FILE*. This patch refactors and
renames that function to make it clear what it's really for and
why nobody else should use it.
Both File::TakeStreamAndClear() and the FILE* typemaps will be
removed in later patches after a suitable replacement is in place.
Reviewers: JDevlieghere, jasonmolenda, labath
Reviewed By: labath
Differential Revision: https://reviews.llvm.org/D68160