[lldb] Have Host::RunShellCommand ret stderr & stdout seperately (#184548)

Host::RunShellCommand takes a std::string *command_output argument and a
bool hide_stderr=false defaulted argument. If the shell command returns
stderr and stdout text, it is intermixed in the same command_output,
unless hide_stderr=true.

In SymbolLocatorDebugSymbols::DownloadObjectAndSymbolFile we call an
external program to find a binary and dSYM by uuid, and the external
program returns a plist (xml) output. In some cases, it printed a
(harmless) warning message to stderr, and then a complete plist output
to stdout. We attempt to parse the combination of these two streams, and
the parse fails - we don't get the output.

This patch removes hide_stderr and instead adds a `std::string
*separated_error_output` argument. If `separated_error_output` is
nullptr, output and error texts are returned combined in the
`command_output` argument. If a std::string object address is provided
for `separated_error_output`, then standard error output is separated
into this string. A caller which wants the old `hide_stderr=true`
behavior should pass a throwaway std::string object to `RunShellCommand`
and ignore it.

rdar://168621579

GitOrigin-RevId: 1c228a0533ae76bace6a3e3fe1c22fc2858dbf75