[CodeView] Saturate values bigger than supported by APInt.

This fixes an assert firing when compiling code which involves 128 bit
integrals.

This would trigger runtime checks similar to this:
```
Assertion failed: getMinSignedBits() <= 64 && "Too many bits for int64_t", file llvm/include/llvm/ADT/APInt.h, line 1646
```

To get around this, we just saturate those big values.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D105320