[lldb][NFC] Another attempt to fix GCC 5.x compilation

37510f69b4cb8d76064f108d57bebe95984a23ae tried to fix GCC 5.x compilation
by making the enum which is used as a unordered_map key unscoped. However it
seems that in GCC 5.x, enum keys are not supported *at all* in unordered_maps
(at least that's what some trial&error on godbolt tells me). This updates the
workaround to just use an int until GCC 5.x support is dropped.

Author: Teemperor

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.h

@@ -1220,10 +1220,13 @@ class ScratchTypeSystemClang : public TypeSystemClang {
   /// imported types.
   std::unique_ptr<ClangASTSource> m_scratch_ast_source_up;
 
+  // FIXME: GCC 5.x doesn't support enum as map keys.
+  typedef int IsolatedASTKey;
+
   /// Map from IsolatedASTKind to their actual TypeSystemClang instance.
   /// This map is lazily filled with sub-ASTs and should be accessed via
   /// `GetSubAST` (which lazily fills this map).
-  std::unordered_map<IsolatedASTKind, std::unique_ptr<TypeSystemClang>>
+  std::unordered_map<IsolatedASTKey, std::unique_ptr<TypeSystemClang>>
       m_isolated_asts;
 };