This change introduces an experimental `generator_version` attribute to the
`sapi_library()` rule. Version `1` will select the current interface
generator, which is based on libclang and Python. Setting the attribute to
version `2`, will select the newer interface generator written in C++ that
uses a full clang compiler frontend for parsing. Both emit equivalent header
output, differences in parsing and/or edge cases notwithstanding.
The default, as of now, is still the old version `1` generator.
Note: CMake allows to select the new interface generator globally by setting
`SAPI_ENABLE_GENERATOR`.
PiperOrigin-RevId: 438765013
Change-Id: I69c49a6bcf1751724edb0bce5c3b2beea2097138
