After investigating with jason, we've concluded this strange code was written
back in the days where roslyn workspaces and VS documents were not something
you could trust to be in sync.  So we had to have a lot of painful code to try
to detect and bail out when that happened.  This is definitely not necessary
now, and we can just use the simple and correct function we already hve for this
task.

Move helper API up as it will be using functionality in EditorFeatures as part of FAR refactoring work.

The synchronous API still exists for TS to use until they
move over to the entirely async api.

This reverts commit 4f7b313c, reversing
changes made to 6d59ad84.

Fixes #17971