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

[aeubanks]

Summary

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.

[aeubanks]

still trying to figure out how bitcode is parsed to see if this works in all situations

[aeubanks]

currently we write the type table before creating values so I think this should be fine

[nikic]

LGTM

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

case bitc::TYPE_CODE_POINTER: { // POINTER: [pointee type] or
         return error("Invalid type");
       ContainedIDs.push_back(Record[0]);
       ResultTy = PointerType::get(ResultTy, AddressSpace);
       break;
     }
     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];
       ResultTy = PointerType::get(Context, AddressSpace);
       break;
     }
     case bitc::TYPE_CODE_FUNCTION_OLD: {
       // Deprecated, but still needed to read old bitcode files.

llvm/test/Other/force-opaque-ptrs.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --function-signature
 ; RUN: llvm-as --opaque-pointers < %s | llvm-dis --opaque-pointers | FileCheck %s
 ; RUN: llvm-as < %s | llvm-dis --opaque-pointers | FileCheck %s
+; RUN: llvm-as --opaque-pointers < %s | llvm-dis | FileCheck %s
 ; RUN: opt --opaque-pointers < %s -S | FileCheck %s
 ; RUN: verify-uselistorder --opaque-pointers < %s
 
 %ty = type i32*
 
 ; CHECK: @g = external global i16
 @g = external global i16