Identified with misc-include-cleaner.

Better reflect the recent history of the code, and improve readability
for when I have to glance back at this to answer a question.

This reverts commit 740161a9b98c9920dedf1852b5f1c94d0a683af5.

I moved the `ISD` dependencies into the CodeGen portion of the handling,
it's a little awkward but it's the easiest solution I can think of for
now.

This reverts commit c05126bdfc3b02daa37d11056fa43db1a6cdef69.
(llvmorg-19-init-17714-gc05126bdfc3b)
See #99610

Summary:
The LTO pass and LLD linker have logic in them that forces extraction
and prevent internalization of needed runtime calls. However, these
currently take all RTLibcalls into account, even if the target does not
support them. The target opts-out of a libcall if it sets its name to
nullptr. This patch pulls this logic out into a class in the header so
that LTO / lld can use it to determine if a symbol actually needs to be
kept.

This is important for targets like AMDGPU that want to be able to use
`lld` to perform the final link step, but does not want the overhead of
uncalled functions. (This adds like a second to the link time trivially)

lowerInvokeable wasn't updating the returned chain after emitting the
lowerEndEH, which caused SwiftErrorVal-handling code to re-set the DAG
root, and thus accidentally skip the EH_LABEL node it was supposed to
have addeed. After fixing that, a few places needed to be adjusted that
assume the specific shape of the returned DAG.

Fixes: #64826
Fixes: rdar://113994760

Prevents isel errors when trying to lower gc relocate of undef value
(which turns into CopyToReg of TargetConstant). Such relocates may occur
after DCE (e.g. after GVN removes some dead blocks) if there are not
passes like instcombine scheduled after to clean them up.

Fixes #80294

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Co-authored-by: Matt Arsenault <arsenm2@gmail.com>