If we use clang-3.5+ instead of gcc and enable its AddessSanitizer (ASAN),
the gen_lex_hash utility aborts on clang's LeakSanitizer memory leak
checks.

This patch adds memory deallocation to gen_lex_hash.cc to make
LeakSanitizer happy (normally this is superfluous, since gen_lex_has is
a compile-time utility, and OS does the job itself).

Note: for for 5.6 only. Fixed in 5.7 and up by patch for Bug#20720615