MCFixup: Improve location accuracy and remove MCFixup::Loc

Remove the redundant MCFixup::Loc member and instead use MCExpr::Loc to
determine the location for fixups. Previously, many target MCCodeEmitter would
use the beginning of an instruction for fixup locations, which often
resulted in inaccurate column information.

```
// RISCVMCCodeEmitter::getImmOpValue
Fixups.push_back(MCFixup::create(0, Expr, FixupKind, MI.getLoc()));

// X86MCCodeEmitter::emitImmediate
Fixups.push_back(MCFixup::create(static_cast<uint32_t>(CB.size() - StartByte), Expr, FixupKind, Loc));
```

While MCExpr::Loc generally provides more meaningful location data,
tests should avoid over-relying on it. For instance, MCBinaryExpr's
location refers to its operator, and for operands with sigils (like
`$foo`), the location often omits the sigils.

https://llvm-compile-time-tracker.com/compare.php?from=8740ff822d462844506134bb7c425e1778518b95&to=831a11f75d22d64982b13dba132d656ac8567612&stat=instructions%3Au

I've also considered removing MCExpr::Loc (revert
https://reviews.llvm.org/D28861), but we'd lose too much information.
It's also difficult to carry location information to improve location
tracking in target MCCodeEmitter.

This change utilizes previous MCExpr::Loc improvement like
7e3e2e1b8c6ff21e68782a56164139cca334fcf3
7b517cf743f112f980cf6a4d6e6190c2a5b3e451

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