SE-0025: Parsing and basic completion for 'fileprivate'. (#3391)

Right now 'fileprivate' is parsed as an alias for 'private' (or
perhaps vice versa, since the semantics of 'private' haven't changed
yet). This allows us to migrate code to 'fileprivate' without waiting
for the full implementation.

lib/IDE/CodeCompletion.cpp

@@ -4062,8 +4062,9 @@ public:
 
     bool missingDeclIntroducer = !hasVarIntroducer && !hasFuncIntroducer;
     bool missingAccess = !isKeywordSpecified("private") &&
-                         !isKeywordSpecified("public") &&
-                         !isKeywordSpecified("internal");
+                         !isKeywordSpecified("fileprivate") &&
+                         !isKeywordSpecified("internal") &&
+                         !isKeywordSpecified("public");
     bool missingOverride = Reason == DeclVisibilityKind::MemberOfSuper &&
                            !isKeywordSpecified("override");