[FileCheck]] Canonicalize caret location testing

Summary:
Testing of caret location in diagnostic message is currently made with
CHECK directive with the following general format:
CHECK: {{^         \^$}}

James Henderson suggested the following would be more readable:
CHECK: {{^}}         ^{{$}}

and when whole lines can be matched (as is the case for command-line
testing where error messages do not include path):
CHECK:         ^
using the option --match-full-lines.

This commit implements these 2 changes on all existing caret position
tests. It also aligns the caret to the character it is trying to match
in the above line.

Reviewers: jhenderson, chandlerc, jdenny, probinson, grimar, arichardson, rnk

Subscribers: JonChesterfield, rogfer01, hfinkel, kristina, rnk, tra, arichardson, grimar, dblaikie, probinson, llvm-commits, hiraditya

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64923

llvm-svn: 366861