Fix respecting LIBOMP_LLVM_LIT_EXECUTABLE as full path

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:

> 3. 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.

Differential Revision: https://reviews.llvm.org/D24710

llvm-svn: 281887