[XRay][tools] Handle "no subcommand" case for llvm-xray

Summary:
Currently the llvm-xray commandline tool fails to handle the case for
when no subcommand is provided in a graceful manner. This fixes that to
print the help message explaining the subcommands and the available
options.

Reviewers: pcc, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31409

llvm-svn: 298975