[ARM] Tighten up CHECK lines in a test

These tests checked for the line number without a leading ":", so for example,
a missed diagnostic on line 123 could match one on line 1123, 2123, etc,
desynchronising the test for hundreds of lines.

This couldn't cause it to incorrectly pass or fail, but made it hard to track
down test failures.

Differential revision: https://reviews.llvm.org/D39238

llvm-svn: 316442