[sil] Teach the SILVerifier how to validate that a builtin insts are always mapped to a known builtin.

The builtin instruction in SIL identifies the operation that it performs via a
simple string name. This commit changes the SILVerifier so that we validate that
any builtin inst can be mapped via its string name to a /true/ builtin value
decl.

Just tightening up the IR model.

Author: gottesmm

lib/SIL/SILVerifier.cpp

@@ -1630,8 +1630,16 @@ class SILVerifier : public SILVerifierBase<SILVerifier> {
 
   void checkBuiltinInst(BuiltinInst *BI) {
     // Check for special constraints on llvm intrinsics.
-    if (BI->getIntrinsicInfo().ID != llvm::Intrinsic::not_intrinsic)
+    if (BI->getIntrinsicInfo().ID != llvm::Intrinsic::not_intrinsic) {
       verifyLLVMIntrinsic(BI, BI->getIntrinsicInfo().ID);
+      return;
+    }
+
+    // At this point, we know that we have a Builtin that is a Swift Builtin
+    // rather than an llvm intrinsic. Make sure our name corresponds to an
+    // actual ValueDecl. Otherwise, we have an invalid builtin.
+    require(getBuiltinValueDecl(BI->getModule().getASTContext(), BI->getName()),
+            "Invalid builtin name?!");
   }
 
   void checkFunctionRefBaseInst(FunctionRefBaseInst *FRI) {