[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