[BoundsSafety] Unify ParseLexedCAttribute into ParseLexedAttribute

ParseLexedCAttribute was copied from ParseLexedAttribute primarily to handle C code. Ideally, these should be unified into a single ParseLexedAttribute function. This is tracked upstream in https://github.com/llvm/llvm-project/issues/93263; however, the change should first be made in swiftlang, which contains the full -fbounds-safety implementation, to ensure compatibility with logic not yet present upstream. Once validated, the change can then be upstreamed to llvm/llvm-project.