[ClangImporter] Use the current version to vend enum constant names.

The compiler itself no longer uses this API but the debugger does,
in order to pretty-print option sets.

The normal way to test this would be to add an LLDB-side test that
uses a framework with versioned API notes. Unfortunately I can't
think of a straightforward way to test it Swift-side.

Author: jrose-apple

include/swift/ClangImporter/ClangImporter.h

@@ -260,8 +260,7 @@ class ClangImporter final : public ClangModuleLoader {
   /// The return value may be an empty identifier, in which case the enum would
   /// not be imported.
   ///
-  /// This is mostly an implementation detail of the importer, but is also
-  /// used by the debugger.
+  /// This is not used by the importer itself, but is used by the debugger.
   Identifier getEnumConstantName(const clang::EnumConstantDecl *enumConstant);
 
   /// Writes the mangled name of \p clangDecl to \p os.

lib/ClangImporter/ImportDecl.cpp

@@ -7570,7 +7570,7 @@ ClangImporter::Implementation::getSpecialTypedefKind(clang::TypedefNameDecl *dec
 
 Identifier
 ClangImporter::getEnumConstantName(const clang::EnumConstantDecl *enumConstant){
-  return Impl.importFullName(enumConstant, ImportNameVersion::Swift3)
+  return Impl.importFullName(enumConstant, Impl.CurrentVersion)
       .getDeclName()
       .getBaseName();
 }