[Module] Simplify explicit clang module loading

When using explicit module build, there is no need to check top level
module map to see if the module exists or not, since dependency scanning
already pulled in all needed modules. Furthermore, when using clang
include tree, the module maps are not available through FS for this
search. Just directly try to load modules when using explicit module
build.

Author: cachemeifyoucan

lib/ClangImporter/ClangImporter.cpp

@@ -1964,12 +1964,16 @@ ModuleDecl *ClangImporter::Implementation::loadModuleClang(
   auto &clangHeaderSearch = getClangPreprocessor().getHeaderSearchInfo();
   auto realModuleName = SwiftContext.getRealModuleName(path.front().Item).str();
 
-  // Look up the top-level module first, to see if it exists at all.
-  clang::Module *clangModule = clangHeaderSearch.lookupModule(
-      realModuleName, /*ImportLoc=*/clang::SourceLocation(),
-      /*AllowSearch=*/true, /*AllowExtraModuleMapSearch=*/true);
-  if (!clangModule)
-    return nullptr;
+  // For explicit module build, module should always exist but module map might
+  // not be exist. Go straight to module loader.
+  if (Instance->getInvocation().getLangOpts()->ImplicitModules) {
+    // Look up the top-level module first, to see if it exists at all.
+    clang::Module *clangModule = clangHeaderSearch.lookupModule(
+        realModuleName, /*ImportLoc=*/clang::SourceLocation(),
+        /*AllowSearch=*/true, /*AllowExtraModuleMapSearch=*/true);
+    if (!clangModule)
+      return nullptr;
+  }
 
   // Convert the Swift import path over to a Clang import path.
   SmallVector<std::pair<clang::IdentifierInfo *, clang::SourceLocation>, 4>
@@ -2024,7 +2028,7 @@ ModuleDecl *ClangImporter::Implementation::loadModuleClang(
 
   // Now load the top-level module, so that we can check if the submodule
   // exists without triggering a fatal error.
-  clangModule = loadModule(clangPath.front(), clang::Module::AllVisible);
+  auto clangModule = loadModule(clangPath.front(), clang::Module::AllVisible);
   if (!clangModule)
     return nullptr;