[CMake] Add configuration dirs as potential locations for llvm-lit and llvm-tblgen in standalone builds
If the provided LLVM build-tree used a multi-configuration generator like Xcode, LLVM_TOOLS_BINARY_DIR will have a generator-specific placeholder to express CMAKE_CFG_INTDIR. Thus llvm-lit and llvm-tblgen won't be found.
D62878 exports the actual configuration types so we can fix the path and add them to the search paths for find_program().
Reviewers: xiaobai, labath, stella.stamenova
Reviewed By: xiaobai, stella.stamenova
Subscribers: mgorny, lldb-commits
Differential Revision: https://reviews.llvm.org/D62879