Thanks to the effort of @RoseZhang03 and @aaryanshukla under the
guidance of
@michaelrj-google and @amykhuang, we now have newhdrgen and no longer
have a
dependency on TableGen and thus LLVM in order to start bootstrapping a
full
build.

This PR removes:
- LIBC_HDRGEN_EXE; the in tree newhdrgen is the only hdrgen that can be
used.
- LIBC_USE_NEW_HEADER_GEN; newhdrgen is the default and only option.
- LIBC_HDRGEN_ONLY; there is no need to have a distinct build step for
old
  hdrgen.
- libc-api-test and libc-api-test-tidy build targets.
- Deletes all .td files.

It does not rename newhdrgen to just hdrgen. Will follow up with a
distinct PR
for that.

Link: #117209
Link: #117254
Fixes: #117208

Change `getValueAsListOfDefs` to return a vector of const Record
pointer, and remove `getValueAsListOfConstDefs` that was added as a
transition aid.

This is a part of effort to have better const correctness in TableGen
backends:


https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089

Fix LibC TableGen build failure caused by
https://github.com/llvm/llvm-project/pull/110032

Migrate libc-hdrgen backend to use const RecordKeeper

They are added as entrypoint object targets. The header-gen
infrastructure has been extended to enable handling standard required
global objects. The libc-api-test has also been extended to verify the
global object declarations.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D126329

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D117855

This tool will be used to generate C wrappers for the C++ LLVM libc
implementations. This change does not hook this tool up to anything yet.
However, it can be useful for cases where one does not want to run the
objcopy step (to insert the C symbol in the object file) but can make use
of LTO to eliminate the cost of the additional wrapper call. This can be
relevant for certain downstream platforms. If this tool can benefit other
libc platforms in general, then it can be integrated into the build system
with options to use or not use the wrappers. An example of such a
platform is CUDA.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D84848