parser: Allow Lm (MODIFIER LETTER) category in identifiers

Authored by thomie.

Description

parser: Allow Lm (MODIFIER LETTER) category in identifiers

Easy fix in the parser to stop regressions, due to Unicode 7.0 changing
the classification of some prior code points.

Signed-off-by: Austin Seipp <austin@well-typed.com>

Test Plan: tests/parser/should_compile/T10196.hs

Reviewers: hvr, austin, bgamari

Reviewed By: austin, bgamari

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D969

GHC Trac Issues: Trac #10196