[FileCheck] Add support for hex alternate form in FileCheck

Add printf-style alternate form flag to prefix hex number with 0x when
present. This works on both empty numeric expression (e.g. variable
definition from input) and when matching a numeric expression. The
syntax is as follows:

[[#%#<precision specifier><format specifier>, ...]

where <precision specifier> and <format specifier> are optional and ...
can be a variable definition or not with an empty expression or not.

This feature was requested in https://reviews.llvm.org/D81144#2075532
for llvm/test/MC/ELF/gen-dwarf64.s

Reviewed By: jdenny

Differential Revision: https://reviews.llvm.org/D97845