[CSApply] Don't try to transform editor placeholder if it's type is invalid

CSDiag could re-typecheck closure or other expression which has editor
placeholder inside allowing type variables be bound to unresolved
type, which doesn't really form a valid solution to be applied to AST.
So we need to guard against trying to transform placeholder into a call
to `_undefined` in such case, otherwise in asserts build it's going to
crash with an assert but in release build it would crash in some other
place e.g. xSILGen or trying to type-check captures.

Resolves: rdar://problem/48937223
Resolves: rdar://problem/51599529

Author: xedin

lib/Sema/CSApply.cpp

@@ -3897,6 +3897,19 @@ namespace {
       simplifyExprType(E);
       auto valueType = cs.getType(E);
 
+      // TODO(diagnostics): Once all of the diagnostics are moved to
+      // new diagnostics framework this check could be eliminated.
+      //
+      // Only way for this to happen is CSDiag try to re-typecheck
+      // sub-expression which contains this placeholder with
+      // `AllowUnresolvedTypeVariables` flag set.
+      //
+      // A better solution could be to replace placeholders with this
+      // implicit call early on and type-check that call together with
+      // the rest of the constraint system.
+      if (valueType->hasUnresolvedType())
+        return nullptr;
+
       auto &tc = cs.getTypeChecker();
       auto &ctx = tc.Context;
       // Synthesize a call to _undefined() of appropriate type.

validation-test/compiler_crashers_2_fixed/0196-rdar48937223.swift

@@ -0,0 +1,11 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P {}
+
+func fn<T, U: P>(_ arg1: T, arg2: (T) -> U) {}
+
+func test(str: String) {
+  fn(str) { arg in
+    <#FOO#> // expected-error {{editor placeholder in source file}}
+  }
+}