remove my hacks that aggressively threw away multiple

remove my hacks that aggressively threw away multiple
instantiation history in an effort to speed up c99-intconst-1.c.
Now that multiple nested instantiations are allowed, we just
make them and don't pay the cost of lookups.  With the other
changes that went in before this, reverting this is actually
a speedup for c99-intconst-1.c, speeding it up from 1.96s to 1.80s,
and preserves much better loc info.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63036 91177308-0d34-0410-b5e6-96231b3b80d8