[clang] [Serialization] Downgrade error to warning for inconsistent language flags (PR #117840)

[Boris Kolpackov via cfe-commits]

boris-kolpackov wrote:

> Maybe then we could adapt the Fortran/ASM convention where an uppercase extension means "I need preprocessing" and lowercase doesn't. (tongue lightly in cheek…)

>From https://build2.org/build2/doc/build2-build-system-manual.xhtml :

> The `preprocessed` property indicates the degree of preprocessing the module unit requires and is used to optimize module compilation. Valid values are `none` (not preprocessed), `includes` (no #include directives in the source), `modules` (as above plus no module declarations depend on the preprocessor, for example, `#ifdef`, etc.), and `all` (the source is fully preprocessed). Note that for `all` the source may still contain comments and line continuations.

https://github.com/llvm/llvm-project/pull/117840