These are identified by misc-include-cleaner.  I've filtered out those
that break builds.  Also, I'm staying away from llvm-config.h,
config.h, and Compiler.h, which likely cause platform- or
compiler-specific build failures.

The format is: `!instances<T>([regex])`.
    
This operator produces a list of records whose type is `T`. If
`regex` is provided, only records whose name matches the regular
expression `regex` will be included. The format of `regex` is ERE
(Extended POSIX Regular Expressions).

The grammar is `!match(str, regex)` and this operator produces 1
if the `str` matches the regular expression `regex`.

The format of `regex` is ERE (Extended POSIX Regular Expressions).

Do not use the PrintFatalError diagnostic machinery for conditions that
can never happen with any input.

This is just a minor cleanup and a small step in the direction of using
LLVM containers in preference to STL containers in lib/TableGen.

There are cases (like in an upcoming patch to MLIR's `Property` class)
where the ? value is a useful null value. However, existing predicates
make ti difficult to test if the value in a record one is operating is ?
or not.

This commit adds the !initialized predicate, which is 1 on concrete,
non-? values and 0 on ?.

---------

Co-authored-by: Akshat Oke <Akshat.Oke@amd.com>

Change LLVM Asm and TableGen Lexer/Parser error messages to begin with
lower case.

Add a !listflatten operator that will transform an input list of type
`list<list<X>>` to `list<X>` by concatenating elements of the
constituent lists of the input argument.

- Replace use of std::isalpha, std::isdigit, std:isxdigit with LLVM's
StringExtras versions, to avoid possibly locale dependent behavior (e.g.
glibc).
- Create helper function for common checks for valid identifier
characters.