MC: Allow .set to reassign non-MCConstantExpr expressions
GNU Assembler supports symbol reassignment via .set, .equ, or =.
However, LLVM's integrated assembler only allows reassignment for
MCConstantExpr cases, as it struggles with scenarios like:
```
.data
.set x, 0
.long x // reference the first instance
x = .-.data
.long x // reference the second instance
.set x,.-.data
.long x // reference the third instance
```
Between two assignments binds, we cannot ensure that a reference binds
to the earlier assignment. We use MCSymbol::IsUsed and other conditions
to reject potentially unsafe reassignments, but certain MCConstantExpr
uses could be unsafe as well.
This patch enables reassignment by cloning the symbol upon reassignment
and updating the symbol table. Existing references to the original
symbol remain unchanged, and the original symbol is excluded from the
emitted symbol table.
