[libc-commits] [libc] [libc] Add `-Wsign-conversion` & un-reverts `-Wconversion` (PR #129811)

[Vinay Deshmukh via libc-commits]

vinay-deshmukh wrote:

Pinging reviewers from: https://github.com/llvm/llvm-project/pull/127523, and https://github.com/llvm/llvm-project/pull/129771
 (and who have fixed/revert the previous PR for `-Wconversion`) so if there's any additional bugs, it can be caught before merge


@michaelrj-google @Caslyn @frobtech @lntue

https://github.com/llvm/llvm-project/pull/129811