[lldb] Reland "Fix UB in half2float" to fix the ubsan bot.

This relands part of the UB fix in 4b074b49.
The original commit also added some additional tests that uncovered some
other issues (see D102845). I landed all the passing tests in
48780527 and this patch is now just fixing
the UB in half2float. See D102846 for a proposed rewrite of the function.

Original commit message:

  The added DumpDataExtractorTest uncovered that this is lshifting a negative
  integer which upsets ubsan and breaks the sanitizer bot. This patch just
  changes the variable we shift to be unsigned.