[FileSystem] Remove OpenFlags param from several functions.

There was only one place in the entire codebase where a non
default value was being passed, and that place was already hidden
in an implementation file.  So we can delete the extra parameter
and all existing clients continue to work as they always have,
while making the interface a bit simpler.

Differential Revision: https://reviews.llvm.org/D47789

llvm-svn: 334046