[Explicit Module Builds] Add 'ClangImporter' setting for explicitly-built modules

This will mean that '-disable-implicit-swift-modules' also automatically implies two things:
1. Clang modules must also be explicit, and the importer's clang instance will get '-fno-implicit-modules' and '-fno-implicit-module-maps'
2. The importer's clang instance will no longer get a '-fmodules-cache-path=', since it is not needed in explicit builds

lib/Frontend/CompilerInvocation.cpp

@@ -1460,6 +1460,9 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
     Opts.PCHDisableValidation |= Args.hasArg(OPT_pch_disable_validation);
   }
 
+  if (FrontendOpts.DisableImplicitModules)
+    Opts.DisableImplicitClangModules = true;
+
   Opts.ValidateModulesOnce |= Args.hasArg(OPT_validate_clang_modules_once);
   if (auto *A = Args.getLastArg(OPT_clang_build_session_file))
     Opts.BuildSessionFilePath = A->getValue();