Sema: Make isDeclarationUnavailable() static.

It's now unused outside of `TypeCheckAvailability.cpp`.

Author: tshortli

lib/Sema/TypeCheckAvailability.cpp

@@ -1478,7 +1478,7 @@ AvailabilityRange TypeChecker::overApproximateAvailabilityAtLocation(
   return availabilityAtLocation(loc, DC, MostRefined).getPlatformRange();
 }
 
-bool TypeChecker::isDeclarationUnavailable(
+static bool isDeclarationUnavailable(
     const Decl *D, const DeclContext *referenceDC,
     llvm::function_ref<AvailabilityRange()> getAvailabilityRange) {
   ASTContext &Context = referenceDC->getASTContext();

lib/Sema/TypeChecker.h

@@ -1032,12 +1032,6 @@ diagnosticIfDeclCannotBePotentiallyUnavailable(const Decl *D);
 /// is allowed.
 std::optional<Diagnostic> diagnosticIfDeclCannotBeUnavailable(const Decl *D);
 
-/// Same as \c checkDeclarationAvailability but doesn't give a reason for
-/// unavailability.
-bool isDeclarationUnavailable(
-    const Decl *D, const DeclContext *referenceDC,
-    llvm::function_ref<AvailabilityRange()> getAvailabilityRange);
-
 /// Checks whether a declaration should be considered unavailable when
 /// referred to at the given location and, if so, returns the unmet required
 /// version range. Returns None is the declaration is definitely available.