[Verifier] Add verification logic for GlobalIFuncs

Verify that the resolver exists, that it is a defined
Function, and that its return type matches the ifunc's
type. Add corresponding check to BitcodeReader, change
clang to emit the correct type, and fix tests to comply.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D112349

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -2279,7 +2279,11 @@ Error BitcodeReader::resolveGlobalAndIndirectSymbolInits() {
           return error("Alias and aliasee types don't match");
         GA->setAliasee(C);
       } else if (auto *GI = dyn_cast<GlobalIFunc>(GV)) {
-        GI->setResolver(C);
+        Type *ResolverFTy =
+            GlobalIFunc::getResolverFunctionType(GI->getValueType());
+        // Transparently fix up the type for compatiblity with older bitcode
+        GI->setResolver(
+            ConstantExpr::getBitCast(C, ResolverFTy->getPointerTo()));
       } else {
         return error("Expected an alias or an ifunc");
       }