Sema: Fix regression with -experimental-skip-non-inlinable-function-bodies-without-types

My recent capture analysis refactoring broke a subtle corner case that wasn't
exercised by the test suite.

If a local inside g() was skipped, but the outer function was not skipped, we would
return the empty list of captures for g(). But if the interface type of g() actually
involves an outer generic parameter type, then the empty capture list did not record
the fact that a generic signature was needed, so we attempted to form a call to the
local function without a generic signature.

Author: slavapestov

lib/Sema/TypeCheckCaptures.cpp

@@ -627,16 +627,16 @@ class FindCapturedVars : public ASTWalker {
 
 CaptureInfo CaptureInfoRequest::evaluate(Evaluator &evaluator,
                                          AbstractFunctionDecl *AFD) const {
-  BraceStmt *body = AFD->getTypecheckedBody();
-
   auto type = AFD->getInterfaceType();
-  if (type->is<ErrorType>() || body == nullptr)
+  if (type->is<ErrorType>())
     return CaptureInfo::empty();
 
   bool isNoEscape = type->castTo<AnyFunctionType>()->isNoEscape();
   FindCapturedVars finder(AFD->getLoc(), AFD, isNoEscape,
                           AFD->isObjC(), AFD->isGeneric());
-  body->walk(finder);
+
+  if (auto *body = AFD->getTypecheckedBody())
+    body->walk(finder);
 
   if (!AFD->isObjC()) {
     finder.checkType(type, AFD->getLoc());

test/SILGen/skip_local_function_bodies.swift

@@ -0,0 +1,16 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -experimental-skip-non-inlinable-function-bodies-without-types %s -emit-module-path %t/skip_local_function_bodies.swiftmodule
+
+public protocol P {
+    init()
+}
+
+extension P {
+    public func f() {
+        typealias T = Self
+
+        func g(_ x: T) {}
+	g(self)
+    }
+}
+