CUDA/HIP: Use kernel name to map to symbol

Currently CGCUDANV uses an llvm::Function as a key to map kernels to a
symbol in host code.  HIP adds one level of indirection and uses the
llvm::Function to map to a global variable that will be initialized to
the kernel stub ptr.

Unfortunately there is no garantee that the llvm::Function created
by GetOrCreateLLVMFunction will be the same.  In fact, the first
time we encounter GetOrCrateLLVMFunction for a kernel, the type
might not be completed yet, and the type of llvm::Function will be
a generic {}, since the complete type is not required to get a symbol
to a function.  In this case we end up creating two global variables,
one for the llvm::Function with the incomplete type and one for the
function with the complete type. The first global variable will be
declared by not defined, resulting in a linking error.

This change uses the mangled name of the llvm::Function as key in the
KernelHandles map, in this way the same llvm::Function will be
associated to the same kernel handle even if they types are different.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D140663