Frontend: Allow -emit-module-interface with -resolve-imports

Experimental mode for generating module interfaces without running
primary file type-checking. The idea is that the ASTPrinter triggers
requests to only do the minimum amount of type checking work
possible while printing the interface for a module's public
declarations.

These requests may emit diagnostics, but the ASTPrinter should in
theory already be robust against invalid code.

Author: slavapestov

lib/Frontend/FrontendOptions.cpp

@@ -705,7 +705,6 @@ bool FrontendOptions::canActionEmitInterface(ActionType action) {
   switch (action) {
   case ActionType::NoneAction:
   case ActionType::Parse:
-  case ActionType::ResolveImports:
   case ActionType::DumpParse:
   case ActionType::DumpInterfaceHash:
   case ActionType::DumpAST:
@@ -727,6 +726,7 @@ bool FrontendOptions::canActionEmitInterface(ActionType action) {
   case ActionType::ScanDependencies:
   case ActionType::PrintFeature:
     return false;
+  case ActionType::ResolveImports:
   case ActionType::Typecheck:
   case ActionType::MergeModules:
   case ActionType::EmitModuleOnly:

test/ModuleInterface/resolve-imports.swift

@@ -0,0 +1,15 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -resolve-imports %s -emit-module-interface-path %t/main.swiftinterface -enable-library-evolution
+// RUN: %FileCheck %s < %t/main.swiftinterface
+
+// CHECK: import Swift
+
+// CHECK: public func f() -> Swift.Int
+public func f() -> Int {}
+
+// Deliberate semantic errors to ensure private and internal declarations don't
+// get type checked.
+private func bad1(_: NotAThing) -> DoesNotExist {}
+
+internal typealias Bad1 = NotEvenReal.DefinitelyNot