Use .clang-format-ignore

Remove previously added clang-format {off,on} markers from imported code, and
add optional.hpp to the list as well.

.clang-format-ignore

@@ -0,0 +1,3 @@
+jbmc/src/miniz/miniz.cpp
+src/nonstd/optional.hpp
+unit/catch/catch.hpp

jbmc/src/miniz/miniz.cpp

@@ -24,9 +24,6 @@
  *
  **************************************************************************/
 
-// this is imported code, don't impose formatting rules
-// clang-format off
-
 #ifdef _MSC_VER
 // conversion warnings
 #pragma warning(disable:4242)
@@ -7208,4 +7205,3 @@ mz_bool mz_zip_end(mz_zip_archive *pZip)
 #ifdef __cplusplus
 }
 #endif
-// clang-format on

unit/catch/catch.hpp

@@ -13,9 +13,6 @@
 #error Do not include this file directly, use use_catch.h instead
 #endif
 
-// this is imported code, don't impose formatting rules
-// clang-format off
-
 #ifndef TWOBLUECUBES_SINGLE_INCLUDE_CATCH_HPP_INCLUDED
 #define TWOBLUECUBES_SINGLE_INCLUDE_CATCH_HPP_INCLUDED
 // start catch.hpp
@@ -14367,5 +14364,3 @@ using Catch::Detail::Approx;
 // end catch_reenable_warnings.h
 // end catch.hpp
 #endif // TWOBLUECUBES_SINGLE_INCLUDE_CATCH_HPP_INCLUDED
-
-// clang-format on