[RISCV] Rename VariantKind to Specifier

Follow the X86 and Mips 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.

Author: MaskRay

bolt/lib/Target/RISCV/RISCVMCPlusBuilder.cpp

@@ -33,7 +33,7 @@ class RISCVMCPlusBuilder : public MCPlusBuilder {
               CompFuncTy Comp) const override {
     const auto &RISCVExprA = cast<RISCVMCExpr>(A);
     const auto &RISCVExprB = cast<RISCVMCExpr>(B);
-    if (RISCVExprA.getKind() != RISCVExprB.getKind())
+    if (RISCVExprA.getSpecifier() != RISCVExprB.getSpecifier())
       return false;
 
     return MCPlusBuilder::equals(*RISCVExprA.getSubExpr(),
@@ -468,7 +468,7 @@ class RISCVMCPlusBuilder : public MCPlusBuilder {
     if (!isa<RISCVMCExpr>(ImmExpr))
       return false;
 
-    switch (cast<RISCVMCExpr>(ImmExpr)->getKind()) {
+    switch (cast<RISCVMCExpr>(ImmExpr)->getSpecifier()) {
     default:
       return false;
     case RISCVMCExpr::VK_CALL: