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

llvm-svn: 244989