Implement constexpr vector unary operators +, -, ~ and ! .
- Follow the current constant interpreter. All of our boolean operations
on vector types should be '-1' for the 'truth' type.
- Move the following functions from `Sema` to `ASTContext`, because we
used it in new interpreter.
```C++
QualType GetSignedVectorType(QualType V);
QualType GetSignedSizelessVectorType(QualType V);
```
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Signed-off-by: yronglin <yronglin777@gmail.com>
