[clang] [clang] Document the return value of __builtin_COLUMN (PR #118360)

[via cfe-commits]

GeorgeKA wrote:

Any thoughts about this @frederick-vs-ja ?

> @frederick-vs-ja, were you looking for specific wording for source_location()::current().column(), or does this cover it?



https://github.com/llvm/llvm-project/pull/118360