Introduce lldbassert(x)

We would like it if LLDB never crashed, especially if we never caused LLDB to crash
On the other hand, having assertions can sometimes be useful

lldbassert(x) is the best of both worlds:
- in debug builds, it turns into a regular assert, which is fine because we don't mind debug LLDB to crash on development machines
- in non-debug builds, it emits a message formatted just like assert(x) would, but then instead of crashing, it dumps a backtrace, suggests filing a bug, and keeps running

llvm-svn: 231310