[MLIR][GPUToSPIRV] Passing gpu module name to SPIR-V module

This patch allows to pass the gpu module name to SPIR-V
module during conversion. This has many benefits as we can lookup
converted to SPIR-V kernel in the symbol table.

In order to avoid symbol conflicts, `"__spv__"` is added to the
gpu module name to form the new one.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D86384