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
DavidLoftus
54b7f1345a