Fix memory leak introduced in r237314.

The commit r237314 that implements YAML block parsing
introduced a leak that was caught by the ASAN linux buildbot.
YAML Parser stores its tokens in an ilist, and allocates
tokens using a BumpPtrAllocator, but doesn't call the
destructor for the allocated tokens. R237314 added an 
std::string field to a Token which leaked as the Token's
destructor wasn't called. This commit fixes this leak
by calling the Token's destructor when a Token is being
removed from an ilist of tokens.

llvm-svn: 237389