[Clang importer] Fix a use-after-free in enum info caching.

Also, make the cache kick in between when a module is built and when
it is used. This isn't actually an important case, but it makes it a
whole lot more common to actually see the use-after-free break things.

Fixes rdar://problem/24869070.

Author: DougGregor

lib/ClangImporter/ImportEnumInfo.h

@@ -60,7 +60,7 @@ class EnumInfo {
 
   /// The enum's common constant name prefix, which will be stripped from
   /// constants
-  StringRef constantNamePrefix = StringRef();
+  std::string constantNamePrefix = StringRef();
 
   /// The identifying attribute for specially imported enums
   ///

lib/ClangImporter/ImporterImpl.h

@@ -473,6 +473,8 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
       if (*moduleOpt)
         moduleName = (*moduleOpt)->getTopLevelModuleName();
     }
+    if (moduleName.empty())
+      moduleName = decl->getASTContext().getLangOpts().CurrentModule;
 
     StringRef enumName =
       decl->getDeclName() ? decl->getName()