[Dependency Scanning] Batching Clang Import Scans by Name

[qiongsiwu]

This PR is a work in progress at the moment and intends to show a functional prototype.

This PR teaches the dependency scanner to batch the module names and scan a list of names using one thread, instead
of dispatching each individual module by name to the clang importer. This way, the clang importer can process the
list in one shot, avoiding creating duplicating resource for each individual modules.

This PR works in tandem with llvm/llvm-project#129915.

rdar://136303612

[qiongsiwu]

Note to reviewers:

This PR is functionally correct (in the sense that it introduces no swift test failures). The choice of StringSet<> to partition the scanning results by name is not efficient (I think I should simply use a vector instead). And I have not done refactoring ModuleDependencyScanner::resolveAllClangModuleDependencies. I am working on these items (in addition to resolving the merge conflict). llvm/llvm-project#129915 will need some updates as well to include the necessary changes in clang.