[lldb][NativePDB] Use active bits for constant width (#196104)

Static constants with bit widths less than 16 (e.g. char/unsigned char)
in classes were not available. These constants are still encoded with 16
bits
([here](https://github.com/llvm/llvm-project/blob/dd145eb8997878143f648a7601741f6409330963/llvm/lib/DebugInfo/CodeView/CodeViewRecordIO.cpp#L341-L343))
even though they only need less bits. When we tried to create an
initializer for them, we'd check if the bit width (=16) was small enough
to fit into the target (<16). This would fail.

This was a problem in the following code:
```cpp
struct Foo {
  // width(unsigned char) = 8, but 255 was encoded and read as 16 bit
  static constexpr unsigned char u8_max = 255;
};
```

This PR fixes the issue by using `APSInt::getActiveBits` for unsigned
types and `APSInt::getSignificantBits` for signed types. They get the
number of bits required to encode the value. For example a constant 64
would need 7 bits in unsigned and 8 bits in signed types.