[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