This implements a custom compilation database to conditionally add the correct
language flags to the compiler frontend. Otherwise, a C header might receive
`--std=c++17` and fail.
Note: All headers are always processed in C++ mode. We expect that headers of
well-behaved C libraries contain `#ifdef __cplusplus`/`extern "C" {}` guards.
PiperOrigin-RevId: 435302048
Change-Id: Ib84e6e1f301ba434999846a012b3f8c16884648e
