Bare minimum C API for libwclv

Includes running the validator with specified input, determining
outcome, getting validated source and freeing resources

JSON header printing is disabled until there is C API for kernel info

Errors are still output on the library side
Don't limit reported errors count at clang level

Do that in the CLI binary once it handles error printing
Re-enable printing (a skeleton of) the JSON header

Still missing the guts
Add kernel enumeration to C API, use in WebCLHeader