AST: Pattern binding decls inherit their var decl's availability.

This ensures SIL lowering will avoid generating code for unavailable global
vars when `-unavailable-decl-optimization=complete` is specfied.

Part of rdar://106674022

Author: tshortli

lib/AST/Availability.cpp

@@ -181,6 +181,9 @@ AvailabilityInference::parentDeclForInferredAvailability(const Decl *D) {
       return NTD;
   }
 
+  if (auto *PBD = dyn_cast<PatternBindingDecl>(D))
+    return PBD->getAnchoringVarDecl(0);
+
   // Clang decls may be inaccurately parented rdar://53956555
   if (D->hasClangNode())
     return nullptr;

test/SILGen/unavailable_decl_optimization.swift

@@ -6,11 +6,10 @@
 // CHECK-NO-STRIP: s4Test14globalConstantSbvp
 // CHECK-NO-STRIP: s4Test14globalConstant_WZ
 // CHECK-NO-STRIP: s4Test14globalConstantSbvau
-// FIXME: global var isn't stripped
-// HECK-STRIP-NOT: s4Test14globalConstant_Wz
-// HECK-STRIP-NOT: s4Test14globalConstantSbvp
-// HECK-STRIP-NOT: s4Test14globalConstant_WZ
-// HECK-STRIP-NOT: s4Test14globalConstantSbvau
+// CHECK-STRIP-NOT: s4Test14globalConstant_Wz
+// CHECK-STRIP-NOT: s4Test14globalConstantSbvp
+// CHECK-STRIP-NOT: s4Test14globalConstant_WZ
+// CHECK-STRIP-NOT: s4Test14globalConstantSbvau
 @available(*, unavailable)
 public let globalConstant = true