Fix a problem that left references to undefined labels in C code.

The problem was introduced by my recent change that removed the definitions
of internal labels if those labels started while loops, and all references
to them would be converted into "continue" statements within those loops.
The diff removed the definitions of these labels, but they were still being
declared. Those declarations expand out to nothing in most cases, which is
why I did not notice the problem, but they are used in some situations,
such as when MR_LOWLEVEL_DEBUG is defined, in which case they register
the correspondence between the names of labels and the code addresses
they represent. The problem was that the code that was registering this
correspondence referred to a now-undefined label.

compiler/llds_out_file.m:
   When gathering labels to declare, delete while labels that won't end up
   being defined.

   We used to compute the list of entry and internal labels three times:
   when deciding the list of labels to forward-declare up front, when deciding
   the list of labels to define in each module, and when deciding whether
   a C module defined any labels without layout structures. We now
   do it just once, up front, and record the result for later use.

   Fix some out-of-date comments.

tests/hard_coded/no_refs_to_deleted_labels.{m,exp}:
   A test case derived from the code that brought the problem to my attention.

tests/hard_coded/Mmakefile:
   Enable the new test case for LLDS grades. (The problem isn't relevant
   in other grades.)

tests/hard_coded/Mercury.options:
   Specify -DMR_LOWLEVEL_DEBUG when compiling the new test case, to make
   the test case fail without the bug fix.

compiler/opt_debug.m:
   Fix white space in LLDS dumps.
Merge branch 'master' of https://github.com/Mercury-Language/mercury