"const" is extraneous in the presence of "constexpr" for simple
variables and arrays.

Note that "override" makes "virtual" redundant.

Identified with modernize-use-override.

With the advent of intrinsic-less debug-info, we no longer need to
scatter calls to getPrevNonDebugInstruction around the codebase. Remove
most of them -- there are one or two that have the "SkipPseudoOp" flag
turned on, however they don't seem to be in positions where skipping
anything would be reasonable.

There are no longer debug-info instructions, thus we don't need this
skipping. Horray!

This patch transforms:

  X && *X == Y

to:

  X == Y

where X is of std::optional<T>, and Y is of T or similar.

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.

This patch addresses clang-tidy's readability-const-return-type by
dropping const from the return type while switching to StringRef at
the same time because these functions just return string constants.

Currently BlockAddresses store both the Function and the BasicBlock they
reference, and the BlockAddress is part of the use list of both the
Function and BasicBlock.

This is quite awkward, because this is not really a use of the function
itself (and walks of function uses generally skip block addresses for
that reason). This also has weird implications on function RAUW (as that
will replace the function in block addresses in a way that generally
doesn't make sense), and causes other peculiar issues, like the ability
to have multiple block addresses for one block (with different
functions).

Instead, I believe it makes more sense to specify only the basic block
and let the function be implied by the BB parent. This does mean that we
may have block addresses without a function (if the BB is not inserted),
but this should only happen during IR construction.