Revert an unintentional API ABI break

lldb::LanguageType is used as a parameter in
SBExpressionOptions::SetLanguage(), which actually makes this type API
too. Commit 6eaedbb5 added a
`: uint16_t` to it, which broke binary compatibility for the SBAPI. This
patch reverts to the original enum.

I tried moving the entire enum into include/API, but that created a
cyclic module dependency between API and Utility. To keep things
simple, this just reverts to the original code and adds a warning.

rdar://103415402

Differential Revision: https://reviews.llvm.org/D141087