Use a function attribute (amdgpu-dynamic-vgpr) instead of a subtarget
feature, as requested in #130030.

This function was a workaround used to detect cyclic dependency
(properly resolved by 343428c666f9293ae260bbcf79130562b830b268).
We do not want backends to use it. However, #112251 exposed it to MCExpr
to be reused by AMDGPU. Keep the workaround within AMDGPU to prevent
other backends from accidentally relying on it.

Many targets define MCTargetExpr subclasses just to encode an expression
with a relocation specifier. Create a generic MCSpecifierExpr to be
inherited instead. Migrate M68k and SPARC as examples.

It has been made ineffective by e015626f189dc76f8df9fdc25a47638c6a2f3feb.
This change migrates the users.

Add `llvm::uninitialized_copy` that accepts a range instead of start/end
iterator for the source of the copy.

This MIPS behavior from edb9d84dcc4824865e86f963e52d67eb50dde7f5 (2010)
is obsoleted and misleading. This caused confusion in
https://reviews.llvm.org/D123702 ([NVPTX] Disable parens for identifiers
starting with '$')

Note: $tmp was rejected by AsmParser before
https://reviews.llvm.org/D75111 (2020)

Follow-up to d6fbffa23c84e622735b3e880fd800985c1c0072 . This commit
updates all call sites and removes the argument from the function.

I was wrong last patch. I viewed the `Visited` set purely as a possible
recursion deterrent where functions calling a callee multiple times are
handled elsewhere. This doesn't consider cases where a function is
called multiple times by different callers still part of the same call
graph. New test shows the aforementioned case.

Reapplies #111004, fixes #115562.

Identified with misc-include-cleaner.