[Parse] Return 'const' pointer from AvailabilityMacroArgumentsRequest

Because it shouldn't be modified.

include/swift/AST/ParseRequests.h

@@ -196,7 +196,7 @@ private:
 /// Parse the '-define-availability' arguments.
 class AvailabilityMacroArgumentsRequest
     : public SimpleRequest<AvailabilityMacroArgumentsRequest,
-                           AvailabilityMacroMap *(ASTContext *),
+                           const AvailabilityMacroMap *(ASTContext *),
                            RequestFlags::Cached> {
 public:
   using SimpleRequest::SimpleRequest;
@@ -205,7 +205,8 @@ private:
   friend SimpleRequest;
 
   // Evaluation.
-  AvailabilityMacroMap *evaluate(Evaluator &evaluator, ASTContext *ctx) const;
+  const AvailabilityMacroMap *evaluate(Evaluator &evaluator,
+                                       ASTContext *ctx) const;
 
 public:
   // Caching.

include/swift/AST/ParseTypeIDZone.def

@@ -35,5 +35,5 @@ SWIFT_REQUEST(Parse, EvaluateIfConditionRequest,
               (std::pair<bool, bool>)(SourceFile *, SourceRange, bool), Uncached,
               NoLocationInfo)
 SWIFT_REQUEST(Parse, AvailabilityMacroArgumentsRequest,
-              (AvailabilityMacroMap *)(ASTContext *), Cached,
+              (const AvailabilityMacroMap *)(ASTContext *), Cached,
               NoLocationInfo)

lib/Parse/ParseRequests.cpp

@@ -591,7 +591,7 @@ bool parseAvailabilityMacroDefinition(
 
 } // namespace
 
-AvailabilityMacroMap *
+const AvailabilityMacroMap *
 AvailabilityMacroArgumentsRequest::evaluate(Evaluator &evaluator,
                                             ASTContext *ctx) const {
   SourceManager &SM = ctx->SourceMgr;