[Sema] Inherit ExprAvailabilityWalker from BaseDiagnosticWalker

Simplifies things a tiny amount, and ensures we
don't attempt to walk into non-PatternBindingDecls,
same as other MiscDiagnostic passes. I don't think
this currently makes a difference since I don't
believe we have any interesting cases where a Decl
is nested in an Expr without an intermediate Stmt,
but it's the right thing to do regardless.

Author: hamishknight

lib/Sema/TypeCheckAvailability.cpp

@@ -3561,7 +3561,7 @@ bool diagnoseExplicitUnavailability(
 }
 
 namespace {
-class ExprAvailabilityWalker : public ASTWalker {
+class ExprAvailabilityWalker : public BaseDiagnosticWalker {
   /// Models how member references will translate to accessor usage. This is
   /// used to diagnose the availability of individual accessors that may be
   /// called by the expression being checked.
@@ -3596,11 +3596,6 @@ class ExprAvailabilityWalker : public ASTWalker {
   explicit ExprAvailabilityWalker(const ExportContext &Where)
     : Context(Where.getDeclContext()->getASTContext()), Where(Where) {}
 
-  MacroWalking getMacroWalkingBehavior() const override {
-    // Expanded source should be type checked and diagnosed separately.
-    return MacroWalking::Arguments;
-  }
-
   PreWalkAction walkToArgumentPre(const Argument &Arg) override {
     // Arguments should be walked in their own member access context which
     // starts out read-only by default.