Reinstate "FileCheck [5/12]: Introduce regular numeric variables"

This reinstates r360578 (git e47362c1),
reverted in r360653 (git 00439368),
with a fix for the list added in FileCheck.rst to build without error.

Copyright:
    - Linaro (changes up to diff 183612 of revision D55940)
    - GraphCore (changes in later versions of revision D55940 and
                 in new revision created off D55940)

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

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60385

llvm-svn: 360665