[MLIR Parser] Improve QoI for "expected token" errors

A typical problem with missing a token is that the missing
token is at the end of a line.  The problem with this is that
the error message gets reported on the start of the following
line (which is where the next / invalid token is) which can
be confusing.

Handle this by noticing this case and backing up to the end of
the previous line.

Differential Revision: https://reviews.llvm.org/D125295