go/parser: add ParseExprFrom function

go/parser: add ParseExprFrom function

This is needed for code that relies on having the correct file set
when parsing an expression only. There's currently no other way to
get to the file set otherwise or to invoke the parser correctly to
work on an expression only with a given file set.

Change-Id: I325f174cb34b69284e627f59fe8334efa4eaa45c
Reviewed-on: https://go-review.googlesource.com/10998
Reviewed-by: Alan Donovan <adonovan@google.com>