[CUDA] Change initializer for CUDA device code based on CUDA documentation.

Summary:
According to CUDA documentation, global variables declared with __device__,
__constant__ can be initialized from host code, so mark them as
externally initialized. Because __shared__ variables cannot have an
initialization as part of their declaration and since the value maybe kept
across different kernel invocation, the value of __shared__ is effectively
undefined instead of zero initialized.

Wrongly using zero initializer may cause illegitimate optimization, e.g.
removing unused __constant__ variable because it's not updated in the device
code and the value is initialized with zero.

Test Plan: test/CodeGenCUDA/address-spaces.cu

Patch by Xuetian Weng

Reviewers: jholewinski, eliben, tra, jingyue

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D12241

llvm-svn: 245786