[Host] Return the user's shell from GetDefaultShell

LLDB handles shell expansion by running lldb-argdumper under a shell.
Currently, this is always /bin/sh on POSIX. This potentially leads to
different behavior between lldb and the user's current shell. Here's an
example of different expansions between shells:

$ /bin/bash -c 'echo -config={Options:[{key:foo_key,value:foo_value}]}'
-config={Options:[key:foo_key]} -config={Options:[value:foo_value]}

$ /bin/zsh -c 'echo -config={Options:[{key:foo_key,value:foo_value}]}'
zsh:1: no matches found: -config={Options:[key:foo_key]}

$ /bin/sh -c 'echo -config={Options:[{key:foo_key,value:foo_value}]}'
-config={Options:[key:foo_key]} -config={Options:[value:foo_value]}

$ /bin/fish -c 'echo -config={Options:[{key:foo_key,value:foo_value}]}'
-config=Options:[key:foo_key] -config=Options:[value:foo_value]

To reduce surprises, this patch returns the user's current shell. It
first looks at the SHELL environment variable. If that isn't set, it'll
ask for the user's default shell. Only if that fails, we'll fallback to
/bin/sh, which should always be available.

Differential revision: https://reviews.llvm.org/D68316

llvm-svn: 373644