Modules: Remove ModuleLoader::OtherUncachedFailure, NFC

5cca6223 refactored
CompilerInstance::loadModule, splitting out
findOrCompileModuleAndReadAST, but was careful to avoid making any
functional changes. It added ModuleLoader::OtherUncachedFailure to
facilitate this and left behind FIXMEs asking why certain failures
weren't cached.

After a closer look, I think we can just remove this and simplify the
code. This changes the behaviour of the following (simplified) code from
CompilerInstance::loadModule, causing a failure to be cached more often:

```
  if (auto MaybeModule = MM.getCachedModuleLoad(*Path[0].first))
    return *MaybeModule;
  if (ModuleName == getLangOpts().CurrentModule)
    return MM.cacheModuleLoad(PP.lookupModule(...));
  ModuleLoadResult Result = findOrCompileModuleAndReadAST(...);
  if (Result.isNormal()) // This will be 'true' more often.
    return MM.cacheModuleLoad(..., Module);
  return Result;
```

`MM` here is a ModuleMap owned by the Preprocessor. Here are the cases
where `findOrCompileModuleAndReadAST` starts returning a "normal" failed
result:
- Emitted `diag::err_module_not_found`, where there's no module map
  found.
- Emitted `diag::err_module_build_disabled`, where implicitly building
  modules is disabled.
- Emitted `diag::err_module_cycle`, which detects module cycles in the
  implicit modules build system.
- Emitted `diag::err_module_not_built`, which avoids building a module
  in this CompilerInstance if another one tried and failed already.
- `compileModuleAndReadAST()` was called and failed to build.

The four errors are all fatal, and last item also reports a fatal error,
so it this extra caching has no functionality change... but even if it
did, it seems fine to cache these failed results within a ModuleMap
instance (note that each CompilerInstance has its own Preprocessor and
ModuleMap).

Differential Revision: https://reviews.llvm.org/D101667