[CMake] Use LLVM_RUNTIME_OUTPUT_INTDIR for LLDB exectuable

Apparently there's a difference between using LLVM_RUNTIME_OUTPUT_INTDIR
and LLVM_BINARY_DIR. The former will point to the current binary
directory (i.e. that where lldb is built) while the former will always
point to LLVM's. This was causing trouble for the swift build but should
be a transparent for upstream lldb.

llvm-svn: 336278