Implement ProcessInfo::Dump(), log gdb-remote stub launch

This change implements dumping the executable, triple,
args and environment when using ProcessInfo::Dump().

It also tweaks the way Args::Dump() works so that it prints
a configurable label rather than argv[{index}]={value}. By
default it behaves the same, but if the Dump() method with
the additional arg is provided, it can be overridden. The
environment variables dumped as part of ProcessInfo::Dump()
make use of that.

lldb-server has been modified to dump the gdb-remote stub's
ProcessInfo before launching if the "gdb-remote process" channel
is logged.

llvm-svn: 271312