Implement Core.Negate for float literals (#7616)

Addresses part of issue raised in #7159 by implementing float.negate
builtin for FloatLiteralValues

The following code now compiles:

```
let a: f64 = -1.0;
```

To achieve this I switch the mantissa from being unsigned to signed.
Lexed literals within source file will still always be unsigned, however
it is now possible to create negative FloatLiteralValue constants. Main
non-local changes this causes is:

- All llvm::APInt parsing / printing calls flipped isSigned param
- Zero extension replaced with sign extension
- getActiveBits() replaced with getSignificantBits() for min bit width
calculation

toolchain/sem_ir/builtin_function_kind.cpp

@@ -724,8 +724,8 @@ constexpr BuiltinInfo IntGreaterEq = {
     "int.greater_eq", ValidateSignature<auto(IntT, IntU)->Bool>};
 
 // "float.negate": float negation.
-constexpr BuiltinInfo FloatNegate = {
-    "float.negate", ValidateSignature<auto(SizedFloatT)->SizedFloatT>};
+constexpr BuiltinInfo FloatNegate = {"float.negate",
+                                     ValidateSignature<auto(FloatT)->FloatT>};
 
 // "float.add": float addition.
 constexpr BuiltinInfo FloatAdd = {