Inferred submodule declarations are emitted in DWARF as `DW_TAG_module`s
without `DW_AT_LLVM_include_path`s. Instead the parent DIE will have the
include path. This patch adds support for such setups. Without this, the
`ClangModulesDeclVendor` would fail to `AddModule` the submodules (i.e.,
compile and load the submodules). This would cause errors such as:
```
note: error: Header search couldn't locate module 'TopLevel'
```

The test added here also tests
https://github.com/llvm/llvm-project/pull/141220. Without that patch
we'd fail with:
```
note: error: No module map file in /Users/jonas/Git/llvm-worktrees/llvm-project/lldb/test/API/lang/cpp/decl-from-submodule
```

Unfortunately the embedded clang instance doesn't allow us to use the
decls we find in the modules. But we'll try to fix that in a separate
patch.

rdar://151022173