Fix lit search to correctly respect LIBOMP_LLVM_LIT_EXECUTABLE as full program path.
The variable passed to find_program() is created by CMake as a cache variable, and therefore can be directly overriden by the user. Since this was the design of LIBOMP_LLVM_LIT_EXECUTABLE (as can be deduced from the error messages) and there is no other use of LIT_EXECUTABLE, remove the redundant variable and pass LIBOMP_LLVM_LIT_EXECUTABLE directly to find_program().
Furthermore, the previous code did not work since the HINTS argument specifies more search directories rather than expected full path. Quoting the CMake documentation:
- Search the paths specified by the HINTS option. These should be paths computed by system introspection, such as a hint provided by the location of another item already found. Hard-coded guesses should be specified with the PATHS option.