Return unique_xmalloc_ptr for generate_c_for_variable_locations

Return unique_xmalloc_ptr for generate_c_for_variable_locations

This patch eliminates two cleanups in compile/ by changing
generate_c_for_variable_locations so that it returns a unique_ptr.

gdb/ChangeLog:
        * compile/compile-c-support.c (c_compute_program): Use
        unique_xmalloc_ptr to eliminate cleanup.
        * compile/compile-c-symbols.c (generate_c_for_variable_locations):
        Return a unique_xmalloc_ptr and eliminate cleanup.
        * compile/compile-internal.h (generate_c_for_variable_locations):
        Return unique_xmalloc_ptr and update description.