lldb-test symbols: Add -file argument and the ability to dump global variables in a file

The motivation for this is to be able to Dwarf index ability to look up
variables within a given compilation unit. It also fits in with the
patch in progress at D47939, which will add the ability to look up
funtions using file+line pairs.

The verification of which lldb-test options can be used together was
getting a bit unwieldy, so I moved the logic out into a separate
function.

llvm-svn: 334498