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.

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>

- Fix JSON and detailed record emitters to use const reference and pointers.
- Fix code to use C++ structured bindings and range-based loops, including reverse() range for locations.
- Eliminate `NL` define for "\n".
- Change JSON emitter to populate `instance_list` in an earlier loop over superclasses instead of a separate loop.
- Rename variables in JSON emitter to conform to LLVM naming conventions.
- Eliminate unused headers in detailed record emitter.

This adds a '!loc' field to each record containing the file name and
line number of the record declaration.

Identified with clangd.

This patch also drops "&" that binds to a temporary.

Identified with readability-const-return-type.

Based on the output of include-what-you-use. No other library seems affected by
the new forward declaration.

$ clang++ -E  -Iinclude -I../llvm/include ../llvm/lib/TableGen/*.cpp -std=c++14 -fno-rtti -fno-exceptions | wc -l
before: 795231
after: 750654

Related Discourse thread: https://llvm.discourse.group/t/include-what-you-use-include-cleanup
Differential Revision: https://reviews.llvm.org/D118374

Differential Revision: https://reviews.llvm.org/D93969