[clang][modules] Infer framework modules in explicit builds

This patch enables inferring framework modules in explicit builds in all contexts. Until now, inferring framework modules only worked with `-fimplicit-module-maps` due to this block of code:

```
// HeaderSearch::loadFrameworkModule
  case LMM_InvalidModuleMap:
    // Try to infer a module map from the framework directory.
    if (HSOpts->ImplicitModuleMaps)
      ModMap.inferFrameworkModule(Dir, IsSystem, /*Parent=*/nullptr);
    break;
```

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D113880