[lldb] [cmake] Use install directories for LLVM_* variables

Restore the previous behavior of using install directories for
LLVM_MAIN_INCLUDE_DIR, LLVM_LIBRARY_DIR and LLVM_BINARY_DIR.  The update
from llvm-config to CMake has changed the values of those values to use
LLVM_BUILD_* which is plain wrong and breaks stand-alone builds.
Instead, use the CMake counterparts of the values returned
by llvm-config.

Differential Revision: https://reviews.llvm.org/D57995

llvm-svn: 353925