[libclc] [libclc] Add generic implementation of bitfield_insert/extract,bit_reverse (PR #149070)

[Fraser Cormack via cfe-commits]

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frasercrmck wrote:

It's minor but I'd perhaps suggest naming this file `clc_bitfield_extract_decl.inc` as it's a helper file for differently named declarations, and not declaration a builtin called `__clc_bitfield_extract`.

https://github.com/llvm/llvm-project/pull/149070