Excise "Accessibility" from the compiler (2/3)

"Accessibility" has a different meaning for app developers, so we've
already deliberately excised it from our diagnostics in favor of terms
like "access control" and "access level". Do the same in the compiler
now that we aren't constantly pulling things into the release branch.

This commit changes the 'Accessibility' enum to be named 'AccessLevel'.

lib/PrintAsObjC/PrintAsObjC.cpp

@@ -154,7 +154,7 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
   SmallVector<const FunctionType *, 4> openFunctionTypes;
   const DelayedMemberSet &delayedMembers;
 
-  Accessibility minRequiredAccess;
+  AccessLevel minRequiredAccess;
   bool protocolMembersOptional = false;
 
   Optional<Type> NSCopyingType;
@@ -164,7 +164,7 @@ class ObjCPrinter : private DeclVisitor<ObjCPrinter>,
 
 public:
   explicit ObjCPrinter(ModuleDecl &mod, raw_ostream &out,
-                       DelayedMemberSet &delayed, Accessibility access)
+                       DelayedMemberSet &delayed, AccessLevel access)
     : M(mod), os(out), delayedMembers(delayed), minRequiredAccess(access) {}
 
   void print(const Decl *D) {
@@ -2016,7 +2016,7 @@ class ModuleWriter {
   StringRef bridgingHeader;
   ObjCPrinter printer;
 public:
-  ModuleWriter(ModuleDecl &mod, StringRef header, Accessibility access)
+  ModuleWriter(ModuleDecl &mod, StringRef header, AccessLevel access)
     : M(mod), bridgingHeader(header), printer(M, os, delayedMembers, access) {}
 
   /// Returns true if we added the decl's module to the import set, false if
@@ -2753,7 +2753,7 @@ public:
 
 bool swift::printAsObjC(llvm::raw_ostream &os, ModuleDecl *M,
                         StringRef bridgingHeader,
-                        Accessibility minRequiredAccess) {
+                        AccessLevel minRequiredAccess) {
   llvm::PrettyStackTraceString trace("While generating Objective-C header");
   return ModuleWriter(*M, bridgingHeader, minRequiredAccess).writeToStream(os);
 }