Avoid iteration invalidation issues around MaterializedTemporaryExpr
We risk iterator invalidation issues if we use a DenseMap to hold the
backing storage for an APValue. Instead, BumpPtrAllocate them and
use APValue * as our DenseMap value.
Also, don't assume that MaterializedGlobalTemporaryMap won't regrow
between when we initially perform a lookup and later on when we actually
try to insert into it.
This fixes PR24289.
Differential Revision: http://reviews.llvm.org/D11629