[StreamExecutor] Fix allocateDeviceMemory

Summary:
The return value from PlatformExecutor::allocateDeviceMemory needs to be
converted from Expected<GlobalDeviceMemoryBase> to
Expected<GlobalDeviceMemory<T>> in Executor::allocateDeviceMemory.

A similar bug is also fixed for Executor::allocateHostMemory.

Thanks to jprice for identifying this bug.

Reviewers: jprice, jlebar

Subscribers: parallel_libs-commits

Differential Revision: https://reviews.llvm.org/D23849

llvm-svn: 279658