[FileCheck] Add precision to format specifier

Add printf-style precision specifier to pad numbers to a given number of
digits when matching them if the value is smaller than the given
precision. 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><format specifier>, ...]

where <format specifier> is optional and ... can be a variable
definition or not with an empty expression or not. In the absence of a
precision specifier, a variable definition will accept leading zeros.

Reviewed By: jhenderson, grimar

Differential Revision: https://reviews.llvm.org/D81667