refactor testsuite spawnLldbMi args->exe+args

Currently spawnLldbMi accepts both lldb-mi options and executable to debug as
a single parameter. Split them.

As in D55859 we will need to execute one lldb-mi command before loading the
exe. Therefore we can no longer use the exe as lldb-mi command-line parameter
as then there is no way to execute a command before loading exe specified as
lldb-mi command-line parameter.

LocateExecutableSymbolFileDsym should be static, that is also a little
refactorization.

Differential Revision: https://reviews.llvm.org/D55858

llvm-svn: 349607