[ConstraintSystem] Properly cache type for literal initialization coercions

Since original implicit coercion expression is preserved in AST
it needs to have its simplified type cached in the constraint
system in order for AST to get the correct type when solution
is fully applied.

Resolves: rdar://problem/45415874

Author: xedin

lib/Sema/CSApply.cpp

@@ -3599,6 +3599,7 @@ namespace {
 
         literalInit->setImplicit(false);
 
+        cs.setType(expr, toType);
         // Keep the coercion around, because it contains the source range
         // for the original constructor call.
         return expr;

test/Constraints/rdar45415874.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift
+
+final class A<T> {
+  init(_: T) {}
+}
+
+extension A: ExpressibleByNilLiteral where T: ExpressibleByNilLiteral {
+  convenience init(nilLiteral: ()) {
+    self.init(nil)
+  }
+}
+
+struct B {
+  var foo: A<B?> = A(nil)
+}