[OpaquePointers][BitcodeReader] Enable -opaque-pointers if we see an opaque pointer type

This allows us to more easily test opaque pointers e.g. in the case of
ThinLTO where we only have to pass -opaque-pointers to the frontend.

Reviewed By: #opaque-pointers, nikic

Differential Revision: https://reviews.llvm.org/D122048

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1883,7 +1883,9 @@ Error BitcodeReader::parseTypeTableBody() {
     case bitc::TYPE_CODE_OPAQUE_POINTER: { // OPAQUE_POINTER: [addrspace]
       if (Record.size() != 1)
         return error("Invalid opaque pointer record");
-      if (Context.supportsTypedPointers())
+      if (LLVM_UNLIKELY(!Context.hasSetOpaquePointersValue())) {
+        Context.enableOpaquePointers();
+      } else if (Context.supportsTypedPointers())
         return error(
             "Opaque pointers are only supported in -opaque-pointers mode");
       unsigned AddressSpace = Record[0];