[clang][clang-scan-deps] Aggregate the full dependency information.

Differential Revision: https://reviews.llvm.org/D70268

This is a recommit of f978ea49 with a fix for the PowerPC failure.

The issue was that:
* `CompilerInstance::ExecuteAction` calls
  `getTarget().adjust(getLangOpts());`.
* `PPCTargetInfo::adjust` changes `LangOptions::HasAltivec`.
* This happens after the first few calls to `getModuleHash`.

There’s even a FIXME saying:
```
  // FIXME: We shouldn't need to do this, the target should be immutable once
  // created. This complexity should be lifted elsewhere.
```

This only showed up on PowerPC because it's one of the few targets that
almost always changes a hashed langopt.

I looked into addressing the fixme, but that would be a much larger
change, and it's not the only thing that happens in `ExecuteAction` that
can change the module context hash. Instead I changed the code to not
call `getModuleHash` until after it has been modified in `ExecuteAction`.