Index: clang/include/clang/Basic/LangOptions.def
===================================================================
--- clang/include/clang/Basic/LangOptions.def
+++ clang/include/clang/Basic/LangOptions.def
@@ -160,6 +160,7 @@
 BENIGN_LANGOPT(CompilingPCH, 1, 0, "building a pch")
 BENIGN_LANGOPT(BuildingPCHWithObjectFile, 1, 0, "building a pch which has a corresponding object file")
 BENIGN_LANGOPT(CacheGeneratedPCH, 1, 0, "cache generated PCH files in memory")
+BENIGN_LANGOPT(PCHInstantiateTemplates, 1, 0, "performing pending template instantiations already while building a pch")
 COMPATIBLE_LANGOPT(ModulesDeclUse    , 1, 0, "require declaration of module uses")
 BENIGN_LANGOPT(ModulesSearchAll  , 1, 1, "searching even non-imported modules to find unresolved references")
 COMPATIBLE_LANGOPT(ModulesStrictDeclUse, 1, 0, "requiring declaration of module uses and all headers to be in modules")
Index: clang/include/clang/Driver/CC1Options.td
===================================================================
--- clang/include/clang/Driver/CC1Options.td
+++ clang/include/clang/Driver/CC1Options.td
@@ -672,6 +672,8 @@
   HelpText<"Disable inclusion of timestamp in precompiled headers">;
 def building_pch_with_obj : Flag<["-"], "building-pch-with-obj">,
   HelpText<"This compilation is part of building a PCH with corresponding object file.">;
+def pch_instantiate_templates : Flag<["-"], "pch-instantiate-templates">,
+  HelpText<"Perform pending template instantiations already while building the PCH.">;
 
 def aligned_alloc_unavailable : Flag<["-"], "faligned-alloc-unavailable">,
   HelpText<"Aligned allocation/deallocation functions are unavailable">;
Index: clang/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- clang/lib/Frontend/CompilerInvocation.cpp
+++ clang/lib/Frontend/CompilerInvocation.cpp
@@ -3223,6 +3223,7 @@
 
   Opts.CompleteMemberPointers = Args.hasArg(OPT_fcomplete_member_pointers);
   Opts.BuildingPCHWithObjectFile = Args.hasArg(OPT_building_pch_with_obj);
+  Opts.PCHInstantiateTemplates = Args.hasArg(OPT_pch_instantiate_templates);
 }
 
 static bool isStrictlyPreprocessorAction(frontend::ActionKind Action) {
Index: clang/lib/Sema/Sema.cpp
===================================================================
--- clang/lib/Sema/Sema.cpp
+++ clang/lib/Sema/Sema.cpp
@@ -981,6 +981,14 @@
                                  LateParsedInstantiations.begin(),
                                  LateParsedInstantiations.end());
     LateParsedInstantiations.clear();
+
+    // FIXME: Instantiating implicit templates already in the PCH breaks some
+    // OpenMP-specific code paths, see https://reviews.llvm.org/D69585 .
+    if(LangOpts.PCHInstantiateTemplates && !LangOpts.OpenMP) {
+      llvm::TimeTraceScope TimeScope("PerformPendingInstantiations",
+                                     StringRef(""));
+      PerformPendingInstantiations();
+    }
   }
 
   DiagnoseUnterminatedPragmaPack();