[LoongArch] Rename VariantKind to Specifier

Follow the X86, Mips, and RISCV renaming.

> "Relocation modifier" suggests adjustments happen during the linker's relocation step rather than the assembler's expression evaluation.
> "Relocation specifier" is clear, aligns with Arm and IBM AIX's documentation, and fits the assembler's role seamlessly.

In addition, rename *MCExpr::getKind, which confusingly shadows the base class getKind.

The parseSpecifier name follows Sparc.

Author: MaskRay