When preamble contains #undef, indexing code finds the matching #define
and uses that during indexing. However, it would only look for local
definitions. If the macro was defined in a module, MacroInfo
would be nullptr and clangd would crash.
This change fixes the crash by looking for definition in module when
that happens. The indexing result is then exactly the same whether
modules are used or not.
The indexing of macros happens for preamble only, so then #undef must be
in the preamble, which is why we need two .h files in a test.
Note that clangd is currently not ready for module support, but this
brings us one step closer.