Expose ProvidePositionalOption as a public API

The motivation is to reuse the key value parsing logic here to
parse instance specific pass options within the context of MLIR.
The primary functionality exposed is the "," splitting for
arrays and the logic for properly handling duplicate definitions
of a single flag.

Patch by: Parker Schuh <parkers@google.com>

Differential Revision: https://reviews.llvm.org/D68294

llvm-svn: 373815