Prefix the name of the calling host function in the name of callee GPU kernel

Summary:
Provide more context to the name of a GPU kernel by prefixing its name with the host function that calls it. E.g. The first kernel called by `gemm` would be `FUNC_gemm_KERNEL_0`.

Kernels currently follow the "kernel_#" (# = 0,1,2,3,...) nomenclature. This patch makes it easier to map host caller and device callee, especially when there are many kernels produced by Polly-ACC.

Reviewers: grosser, Meinersbur, bollu, philip.pfaffe, kbarton!

Reviewed By: grosser

Subscribers: nemanjai, pollydev

Tags: #polly

Differential Revision: https://reviews.llvm.org/D33985

llvm-svn: 307173