[CUDA] Do not allow dynamic initialization of global device side variables.

In general CUDA does not allow dynamic initialization of
global device-side variables. One exception is that CUDA allows
records with empty constructors as described in section E2.2.1 of
CUDA 7.5 Programming guide.

This patch applies initializer checks for all device-side variables.
Empty constructors are accepted, but no code is generated for them.

Differential Revision: http://reviews.llvm.org/D15305

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@259592 91177308-0d34-0410-b5e6-96231b3b80d8