[analyzer] Don't try to evaluate MaterializeTemporaryExpr as a constant.

...and don't consider '0' to be a null pointer constant if it's the
initializer for a float!

Apparently null pointer constant evaluation looks through both
MaterializeTemporaryExpr and ImplicitCastExpr, so we have to be more
careful about types in the callers. For RegionStore this just means giving
up a little more; for ExprEngine this means handling the
MaterializeTemporaryExpr case explicitly.

Follow-up to r180894.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180944 91177308-0d34-0410-b5e6-96231b3b80d8