Introduce SPIR calling conventions.

This implements Section 3.7 from the SPIR 1.2 spec:

    SPIR kernels should use "spir_kernel" calling convention.
    Non-kernel functions use "spir_func" calling convention. All
    other calling conventions are disallowed.

The patch works only for OpenCL source. Any other uses will need
to ensure that kernels are assigned the spir_kernel calling
convention correctly.

llvm-svn: 226548