[CMake] Add configuration dirs as potential locations for llvm-lit and...

[CMake] Add configuration dirs as potential locations for llvm-lit and llvm-tblgen in standalone builds

Summary:
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

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D62879

llvm-svn: 362589