Turn off lldb debug tuning by default for FreeBSD

Summary:
This is the clang part of D15966.  In rL256104, debugger tuning was
added to the clang driver, and again the default for FreeBSD was set to
lldb.  The default needs to be gdb instead.

Reviewers: emaste, probinson

Subscribers: cfe-commits, emaste

Differential Revision: http://reviews.llvm.org/D15967

llvm-svn: 257104