FileCheck [9/12]: Add support for matching formats

FileCheck [9/12]: Add support for matching formats

Summary:
This patch is part of a patch series to add support for FileCheck
numeric expressions. This specific patch adds support for selecting a
matching format to match a numeric value against (ie. decimal, hex lower
case letters or hex upper case letters).

This commit allows to select what format a numeric value should be
matched against. The following formats are supported: decimal value,
lower case hex value and upper case hex value. Matching formats impact
both the format of numeric value to be matched as well as the format of
accepted numbers in a definition with empty numeric expression
constraint.

Default for absence of format is decimal value unless the numeric
expression constraint is non null and use a variable in which case the
format is the one used to define that variable. Conclict of format in
case of several variable being used is diagnosed and forces the user to
select a matching format explicitely.

This commit also enables immediates in numeric expressions to be in any
radix known to StringRef's GetAsInteger method, except for legacy
numeric expressions (ie [[@LINE+<offset>]] which only support decimal
immediates.

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

Reviewed By: jhenderson, arichardson

Subscribers: daltenty, MaskRay, hiraditya, llvm-commits, probinson, dblaikie, grimar, arichardson, kristina, hfinkel, rogfer01, JonChesterfield

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60389