[lldb][cmake] Remove libclang as an lldbBase dependency (NFCI)

It's pulling in all kinds of things it doesn't need (e.g, clang-tidy!).

Eliminating this dependency removes 1056 dependencies from the
'CommandObjectFrame.cpp.o' target and 454 dependencies from the 'lldb'
target. On my machine, this shaves 7 minutes off of a clean build of
lldb.

Thanks to Zachary Turner for pointing out some issues with an earlier
version of this patch!

Differential Revision: https://reviews.llvm.org/D22987

llvm-svn: 279296