[TableGen] Allow 2^63-1 and 2^63-2 as int literals.

These two values correspond to the 'Empty' and 'Tombstone' special
keys defined by DenseMapInfo<int64_t>, which means that neither one
can be used as a key in DenseMap<int64_t, anything>. Hence, if you try
to use either of those values as an int literal, IntInit::get() fails
an assertion when it tries to insert them into its static cache of
int-literal objects.

Fixed by replacing the DenseMap with a std::map, which doesn't intrude
on the space of legal values of the key type.

Reviewers: nhaehnle, hfinkel, javedabsar, efriedma

Reviewed By: efriedma

Subscribers: fhahn, efriedma, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59016

llvm-svn: 355900