File::Clear() -> File::TakeStreamAndClear()

Summary:
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

Subscribers: lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68160