[OpenCL] Allow partial initializer for array and struct

Currently Clang allows partial initializer for C99 but not for OpenCL, e.g.

float a[16][16] = {1.0f, 2.0f};

is allowed in C99 but not allowed in OpenCL.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D25335

llvm-svn: 283891