[llvm] [Support] Add decodeULEB128AndInc/decodeSLEB128AndInc (PR #85739)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The tablegen use case guarantees that there is no error. The `Shift>63` check is pure overhead.
Use cases that require rigid error checking can probably use `llvm/include/llvm/Support/DataExtractor.h` `getULEB128`, but the overhead would be substantial.
At any rate, the error parameters could be added to `*AndInc` but I have found a good use case yet.

For efficient ULEB128 for the primary use cases (debug info, etc), perhaps we should unroll the first iteration.

https://github.com/llvm/llvm-project/pull/85739