[lldb] Improve swift clang importer when explicit module missing

Instead of dropping all explicit modules when found one module is
missing, only drop the ones that are missing. In this case, the implicit
build will only fill in the missing ones, rather than rebuild
everything.

(cherry picked from commit 90f1e38)

Author: cachemeifyoucan

lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp

@@ -1912,29 +1912,26 @@ void SwiftASTContext::AddExtraClangCC1Args(
       GetCompilerInvocation().getClangModuleCachePath().str();
 
   // Remove non-existing modules in a systematic way.
-  bool module_missing = false;
-  auto CheckFileExists = [&](const char *file) {
-    if (!llvm::sys::fs::exists(file)) {
-      std::string warn;
-      llvm::raw_string_ostream(warn)
-          << "Nonexistent explicit module file " << file;
-      AddDiagnostic(eSeverityWarning, warn);
-      module_missing = true;
-    }
+  auto CheckFileExists = [&](const std::string &file) -> bool {
+    if (llvm::sys::fs::exists(file))
+      return true;
+    std::string warn;
+    llvm::raw_string_ostream(warn)
+        << "Nonexistent explicit module file " << file;
+    AddDiagnostic(eSeverityWarning, warn);
+    return false;
   };
-  llvm::for_each(invocation.getHeaderSearchOpts().PrebuiltModuleFiles,
-                 [&](const auto &mod) { CheckFileExists(mod.second.c_str()); });
-  llvm::for_each(invocation.getFrontendOpts().ModuleFiles,
-                 [&](const auto &mod) { CheckFileExists(mod.c_str()); });
-
-  // If missing, clear all the prebuilt module options and switch to implicit
-  // modules build.
-  if (module_missing) {
-    invocation.getHeaderSearchOpts().PrebuiltModuleFiles.clear();
-    invocation.getFrontendOpts().ModuleFiles.clear();
-    invocation.getLangOpts().ImplicitModules = true;
-    invocation.getHeaderSearchOpts().ImplicitModuleMaps = true;
+  for (auto it = invocation.getHeaderSearchOpts().PrebuiltModuleFiles.begin();
+       it != invocation.getHeaderSearchOpts().PrebuiltModuleFiles.end();) {
+    if (!CheckFileExists(it->second))
+      it = invocation.getHeaderSearchOpts().PrebuiltModuleFiles.erase(it);
+    else
+      ++it;
   }
+  invocation.getFrontendOpts().ModuleFiles.erase(
+      llvm::remove_if(invocation.getFrontendOpts().ModuleFiles,
+                      [&](const auto &mod) { return !CheckFileExists(mod); }),
+      invocation.getFrontendOpts().ModuleFiles.end());
 
   invocation.generateCC1CommandLine(
       [&](const llvm::Twine &arg) { dest.push_back(arg.str()); });