[code-completion] Add type context for single-expression functions

Extend the support for single-expression closures to handle
single-expression functions of all kinds. This allows, e.g.

    func foo() -> MyEnum { .<here> }

to complete members of `MyEnum`.

tools/SourceKit/lib/SwiftLang/SwiftLangSupport.h

@@ -74,6 +74,8 @@ namespace SourceKit {
   class Context;
   class NotificationCenter;
 
+  using TypeContextKind = swift::ide::CodeCompletionContext::TypeContextKind;
+
 class SwiftEditorDocument :
     public ThreadSafeRefCountedBase<SwiftEditorDocument> {
 
@@ -162,7 +164,7 @@ class SessionCache : public ThreadSafeRefCountedBase<SessionCache> {
   CompletionSink sink;
   std::vector<Completion *> sortedCompletions;
   CompletionKind completionKind;
-  bool completionHasExpectedTypes;
+  TypeContextKind typeContextKind;
   bool completionMayUseImplicitMemberExpr;
   FilterRules filterRules;
   llvm::sys::Mutex mtx;
@@ -171,11 +173,10 @@ public:
   SessionCache(CompletionSink &&sink,
                std::unique_ptr<llvm::MemoryBuffer> &&buffer,
                std::vector<std::string> &&args, CompletionKind completionKind,
-               bool hasExpectedTypes, bool mayUseImplicitMemberExpr,
+               TypeContextKind typeContextKind, bool mayUseImplicitMemberExpr,
                FilterRules filterRules)
       : buffer(std::move(buffer)), args(std::move(args)), sink(std::move(sink)),
-        completionKind(completionKind),
-        completionHasExpectedTypes(hasExpectedTypes),
+        completionKind(completionKind), typeContextKind(typeContextKind),
         completionMayUseImplicitMemberExpr(mayUseImplicitMemberExpr),
         filterRules(std::move(filterRules)) {}
   void setSortedCompletions(std::vector<Completion *> &&completions);
@@ -184,7 +185,7 @@ public:
   ArrayRef<std::string> getCompilerArgs();
   const FilterRules &getFilterRules();
   CompletionKind getCompletionKind();
-  bool getCompletionHasExpectedTypes();
+  TypeContextKind getCompletionTypeContextKind();
   bool getCompletionMayUseImplicitMemberExpr();
 };
 typedef RefPtr<SessionCache> SessionCacheRef;