AST: Add assertion to BuiltinLiteralExpr::setBuiltinInitializer()

I just spent way too long debugging an issue where a null pointer
was passed here, and SILGen crashed much later.

Author: slavapestov

include/swift/AST/Expr.h

@@ -696,9 +696,7 @@ class BuiltinLiteralExpr : public LiteralExpr {
 
   /// Set the builtin initializer that will be used to construct the
   /// literal.
-  void setBuiltinInitializer(ConcreteDeclRef builtinInitializer) {
-    BuiltinInitializer = builtinInitializer;
-  }
+  void setBuiltinInitializer(ConcreteDeclRef builtinInitializer);
 };
 
 /// The 'nil' literal.

lib/AST/Expr.cpp

@@ -1089,7 +1089,14 @@ StringRef LiteralExpr::getLiteralKindDescription() const {
   llvm_unreachable("Unhandled literal");
 }
 
-IntegerLiteralExpr * IntegerLiteralExpr::createFromUnsigned(ASTContext &C, unsigned value, SourceLoc loc) {
+void BuiltinLiteralExpr::setBuiltinInitializer(
+    ConcreteDeclRef builtinInitializer) {
+  ASSERT(builtinInitializer);
+  BuiltinInitializer = builtinInitializer;
+}
+
+IntegerLiteralExpr * IntegerLiteralExpr::createFromUnsigned(
+    ASTContext &C, unsigned value, SourceLoc loc) {
   llvm::SmallString<8> Scratch;
   llvm::APInt(sizeof(unsigned)*8, value).toString(Scratch, 10, /*signed*/ false);
   auto Text = C.AllocateCopy(StringRef(Scratch));