SILGen: Add explicit 'self' metadata parameter when 'self' is a mutable capture

If a closure uses DynamicSelfType but doesn't capture the 'self' value
itself, we would add a dummy metatype capture so that IRGen can recover
metadata.

Also do this if we do capture a value of DynamicSelfType, but mutably,
that is by address or by box, since IRGen doesn't know how to recover
the metadata in that case.

Fixes <rdar://problem/32288771>.

Author: slavapestov

lib/SIL/TypeLowering.cpp

@@ -2099,7 +2099,11 @@ TypeConverter::getLoweredLocalCaptures(AnyFunctionRef fn) {
 
           // We're capturing a 'self' value with dynamic 'Self' type;
           // handle it specially.
-          if (captureType->getClassOrBoundGenericClass()) {
+          //
+          // However, only do this if its a 'let'; if the capture is
+          // mutable, we're going to be capturing a box or an address.
+          if (captureType->getClassOrBoundGenericClass() &&
+              capturedVar->isLet()) {
             if (selfCapture)
               selfCapture = selfCapture->mergeFlags(capture);
             else

test/SILGen/dynamic_self.swift

@@ -229,11 +229,11 @@ class Z {
     fn0()
 
     var x = self
-    let fn1 = { _ = x }
+    let fn1 = { _ = x; _ = { _ = x } }
     fn1()
 
     var xx = (self, self)
-    let fn2 = { _ = xx }
+    let fn2 = { _ = xx; _ = { _ = xx } }
     fn2()
 
     return self.init()