Follow-up to #141333. Relocation generation called both addReloc and
applyFixup, with the default addReloc invoking shouldForceRelocation,
resulting in three virtual calls. This approach was also inflexible, as
targets needing additional data required extending
`shouldForceRelocation` (see #73721, resolved by #141311).

This change integrates relocation handling into applyFixup, eliminating
two virtual calls. The prior default addReloc is renamed to
maybeAddReloc. Targets overriding addReloc now call their customized
addReloc implementation.

Use MCSpecifierExpr directly and remove the CSKYMCExpr subclass. Define
printImpl within CSKYMCAsmInfo.

Printing an expression is error-prone without a MCAsmInfo argument.
The callers have migrated to MCAsmInfo::printExpr or MCExpr::print.

so that subclasses can provide the appropriate MCAsmInfo to print
MCExpr objects.

At present, llvm/utils/TableGen/AsmMatcherEmitter.cpp constucts a
generic MCAsmInfo.

We introduced VariantKinds after MCSymbolRefExpr::VariantKind and then
deprecated the VariantKind naming in favor of AtSpecifier (#133214).
Rename the function and type to use the recommended convention.

Without newlines at the end, git diff would display:

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

MCAsmLexer.h has been made a forwarder header since #134207

Replace MCSymbol argument with MCValue::AddSym. The minor difference in
.weakref handling is negligible, as our implementation may not fully
align with GAS, and .weakref is not used in practice.