AST: Support ImportDecls in DeclExportabilityVisitor.

Resolves rdar://117438934

Author: tshortli

include/swift/AST/DeclExportabilityVisitor.h

@@ -127,7 +127,7 @@ class DeclExportabilityVisitor
 #define DEFAULT_TO_ACCESS_LEVEL(KIND)                                          \
   bool visit##KIND##Decl(const KIND##Decl *D) {                                \
     static_assert(std::is_convertible<KIND##Decl *, ValueDecl *>::value,       \
-                  "##KIND##Decl must be a ValueDecl");                         \
+                  #KIND "Decl must be a ValueDecl");                           \
     return false;                                                              \
   }
   DEFAULT_TO_ACCESS_LEVEL(NominalType);
@@ -145,12 +145,11 @@ class DeclExportabilityVisitor
   // exportability queries.
 #define UNREACHABLE(KIND)                                                      \
   bool visit##KIND##Decl(const KIND##Decl *D) {                                \
-    llvm_unreachable("unexpected decl kind");                                  \
+    llvm_unreachable("unexpected " #KIND "Decl");                              \
     return true;                                                               \
   }
   UNREACHABLE(Module);
   UNREACHABLE(TopLevelCode);
-  UNREACHABLE(Import);
   UNREACHABLE(PoundDiagnostic);
   UNREACHABLE(Missing);
   UNREACHABLE(MissingMember);
@@ -169,6 +168,7 @@ class DeclExportabilityVisitor
 #define UNINTERESTING(KIND)                                                    \
   bool visit##KIND##Decl(const KIND##Decl *D) { return true; }
   UNINTERESTING(IfConfig);
+  UNINTERESTING(Import);
   UNINTERESTING(PrecedenceGroup);
   UNINTERESTING(EnumCase);
   UNINTERESTING(Operator);

test/SILGen/skip-non-exportable-decls.swift

@@ -2,6 +2,8 @@
 // RUN: %target-swift-frontend -emit-silgen %s -parse-as-library -module-name Test | %FileCheck %s --check-prefixes=CHECK,CHECK-NO-SKIP
 // RUN: %target-swift-frontend -emit-silgen %s -parse-as-library -module-name Test -experimental-skip-non-exportable-decls | %FileCheck %s --check-prefixes=CHECK,CHECK-SKIP
 
+import Swift
+
 // CHECK-NO-SKIP: sil private{{.*}} @$s4Test11privateFunc33_E3F0E1C7B46D05C8067CB98677DE566CLLyyF : $@convention(thin) () -> () {
 // CHECK-SKIP-NOT: s4Test11privateFunc33_E3F0E1C7B46D05C8067CB98677DE566CLLyyF
 private func privateFunc() {}