cmake: provide flag that enables 'log enable --stack' to provide useful...

cmake: provide flag that enables 'log enable --stack' to provide useful file/function info on POSIX systems

Adding the following flag to a cmake line:
-DLLDB_EXPORT_ALL_SYMBOLS=TRUE

will cause all symbols to be exported from liblldb.  This enables the llvm
backtrace mechanism to see and report backtrace symbols properly when using
(lldb) log enable --stack ...

Prior to this change, only the SB API symbols would show up on Linux and other
systems that use a public-symbols-based backtrace lookup mechanism.

log enable --stack ... is a very handy, quick way to understand the flow
of how some log lines are getting hit within lldb without having to hook
up a top-level debugger over your current debug session.

llvm-svn: 250299