[clangd] Fix crash-bug in preamble indexing when using modules.

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 makes clangd ignore any #undef without a matching #define
inside the same TU.

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.

This was previously attempted in
4061d9e4, but had to be reverted due to
broken test. This version fixes that test-only bug by setting a custom module
cache path to avoid re-use of modules across test invocations.

Differential Revision: https://reviews.llvm.org/D85923