[ClangImporter] Allow direct explicit load when implicit module is enabled

Improve clang importor so it can directly load explicit module even
implicit module is enabled. This is a special configuration used by lldb
because lldb sometimes need to load additional modules when binding
external types. This provide the path in clang importer that can load
explicit module without locating the clang module map, while fallback to
module map lookup when implicit module is needed.

Author: cachemeifyoucan

lib/ClangImporter/ClangImporter.cpp

@@ -2261,10 +2261,6 @@ bool ClangImporter::canImportModule(ImportPath::Module modulePath,
 clang::Module *
 ClangImporter::Implementation::lookupModule(StringRef moduleName) {
   auto &clangHeaderSearch = getClangPreprocessor().getHeaderSearchInfo();
-  if (getClangASTContext().getLangOpts().ImplicitModules)
-    return clangHeaderSearch.lookupModule(
-        moduleName, /*ImportLoc=*/clang::SourceLocation(),
-        /*AllowSearch=*/true, /*AllowExtraModuleMapSearch=*/true);
 
   // Explicit module. Try load from modulemap.
   auto &PP = Instance->getPreprocessor();
@@ -2282,8 +2278,13 @@ ClangImporter::Implementation::lookupModule(StringRef moduleName) {
   // If not, try load it.
   auto &PrebuiltModules = Instance->getHeaderSearchOpts().PrebuiltModuleFiles;
   auto moduleFile = PrebuiltModules.find(moduleName);
-  if (moduleFile == PrebuiltModules.end())
+  if (moduleFile == PrebuiltModules.end()) {
+    if (getClangASTContext().getLangOpts().ImplicitModules)
+      return clangHeaderSearch.lookupModule(
+          moduleName, /*ImportLoc=*/clang::SourceLocation(),
+          /*AllowSearch=*/true, /*AllowExtraModuleMapSearch=*/true);
     return nullptr;
+  }
 
   clang::serialization::ModuleFile *Loaded = nullptr;
   if (!Instance->loadModuleFile(moduleFile->second, Loaded))
@@ -2293,20 +2294,8 @@ ClangImporter::Implementation::lookupModule(StringRef moduleName) {
 
 ModuleDecl *ClangImporter::Implementation::loadModuleClang(
     SourceLoc importLoc, ImportPath::Module path) {
-  auto &clangHeaderSearch = getClangPreprocessor().getHeaderSearchInfo();
   auto realModuleName = SwiftContext.getRealModuleName(path.front().Item).str();
 
-  // 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>
       clangPath;