- 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>

DenseSet, SmallPtrSet, SmallSet, SetVector, and StringSet recently
gained C++23-style insert_range.  This patch replaces:

  Dest.insert(Src.begin(), Src.end());

with:

  Dest.insert_range(Src);

This patch does not touch custom begin like succ_begin for now.

Use `auto` when initializing a variable with `cast<>`. Remove some
unneeded `const_cast` (since all Init pointers are now const).

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

- Refactor SetTheory code to use const pointers when possible.
- Use auto for variables initialized using dyn_cast<>.
- Use range based for loops and early continue.

Since `raw_string_ostream` doesn't own the string buffer, it is
desirable (in terms of memory safety) for users to directly reference
the string buffer rather than use `raw_string_ostream::str()`.

Work towards TODO comment to remove `raw_string_ostream::str()`.

This reverts commit cf239c2f1777eb94a4801a086acf1332a7d3cccf.
It has broken tools/llvm-dwarfdump/X86/simplified-template-names.s on
following buildbot:
https://lab.llvm.org/buildbot/#/builders/178/builds/3148

This change modifies the implementation of the format() function
so that vendor forks committed to building with compilers that
support __attribute__((format)) on non-variadic functions can
check the format() function with it.

rdar://84571523

This reverts commit fb1e90ef07fec0d64a05c0b6d41117a5ea3e8344.