[lit] Add the ability to parse regexes in Lit boolean expressions

This patch augments Lit with the ability to parse regular expressions
in boolean expressions. This includes REQUIRES:, XFAIL:, UNSUPPORTED:,
and all other special Lit markup that evaluates to a boolean expression.

Regular expressions can be specified by enclosing them in {{...}},
similarly to how FileCheck handles such regular expressions. The regular
expression can either be on its own, or it can be part of an identifier.
For example, a match expression like {{.+}}-apple-darwin{{.+}} would match
the following variables:

     x86_64-apple-darwin20.0
     arm64-apple-darwin20.0
     arm64-apple-darwin22.0
     etc...

In the long term, this could be used to remove the need to handle the
target triple specially when parsing boolean expressions.

Differential Revision: https://reviews.llvm.org/D104572