- Also eliminate unneeded std::string() around some literal strings.

In Record only store the direct superclasses instead of all
superclasses. getSuperClasses recurses to find all superclasses when
necessary.

This gives a small reduction in memory usage. On lib/Target/X86/X86.td I
measured about 2.0% reduction in total bytes allocated (measured by
valgrind) and 1.3% reduction in peak memory usage (measured by
/usr/bin/time -v).

---------

Co-authored-by: Min-Yih Hsu <min@myhsu.dev>

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).

- Bail out of TableGen if any asserts fail before running the backend. 
- Add asserts to validate that the `Objects` and `Modes` lists for
various `HwModeSelect` subclasses are of same length.
 - Eliminate equivalent check in CodeGenHWModes.cpp

Also use brace initialization and emplace to avoid explicitly 
constructing std::pair, and the same for std::tuple.

The signature of `CheckTemplateArgValues` implements error handling via
the `bool` return type, yet always returned false. The single possible
error case instead used `PrintFatalError,` which exits the program
afterward.

This behavior is undesirable: It prevents any further errors from being
printed and makes TableGen less usable as a library as it crashes the
entire process (e.g. `tblgen-lsp-server`).

This PR therefore fixes the issue by using `Error` instead and returning
true if an error occurred. All callers already perform proper error
handling.

As `llvm-tblgen` exits on error, a test was also added to the LSP to
ensure it exits normally despite the error.

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>

Fixes #118490

Point to the value name, otherwise it implies that the part after the
'=' is the problem.

Before:
```
/tmp/test.td:2:27: error: Value 'FlattenedFeatures' unknown!
  let FlattenedFeatures = [];
                          ^
```
After:
```
/tmp/test.td:2:7: error: Value 'FlattenedFeatures' unknown!
  let FlattenedFeatures = [];
      ^
```