[CMake] In standalone builds, LLVM_BINARY_DIR should point to LLVM's binary directory

Summary: In standalone builds `LLVM_BINARY_DIR` was equal to `LLDB_BINARY_DIR` so far. This is counterintuitive and invalidated the values of `LLDB_DEFAULT_TEST_DSYMUTIL/FILECHECK/COMPILER` etc.

Reviewers: zturner, labath, clayborg, JDevlieghere, stella.stamenova, serge-sans-paille

Reviewed By: labath

Subscribers: mgorny, friss, lldb-commits, #lldb

Differential Revision: https://reviews.llvm.org/D56443

llvm-svn: 350738