[clangd] [Modules] Fixes to correctly handle module dependencies (#142090)

Simple module import dependencies, see
[module_dependencies.test](https://github.com/llvm/llvm-project/compare/main...fleeting-xx:llvm-project:fix_clangd_dependent_modules#diff-5510681cbe5b7ed3a72c5e683184e83fa66e911e9abb0e6670b01b87b3ca7b1a),
were not being correctly handled due to a couple of issues.

- The `MDB.getRequiredModules()` call returned a
`std::vector<std::string>` and all `StringRefs` were to entries in that
temporary value. So the `StringRef` elements in
`getAllRequiredModules()`'s return value were bound to values that went
out of scope.
- `ModulesBuilder::ModulesBuilderImpl::getOrBuildModuleFile()` was
iterating over each module dependency name, but only using the original
module name and path for various checks and module compilation.

In addition to fixing the above issues I added support for Windows paths
in modules.test and added a new unit test, module_dependencies.test,
which demonstrates the failure in the previous state and works correctly
after the fixes have been applied.

Please let me know if I've missed anything.

Co-authored-by: Dan Baker <dan@requires.coffee>

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