[MC] Compute fragment offsets eagerly

This builds on top of commit 9d0754ada5dbbc0c009bcc2f7824488419cc5530
("[MC] Relax fragments eagerly") and relaxes fragments eagerly to
eliminate MCSection::HasLayout and `getFragmentOffset` overhead. The
approach is slightly different from
1a47f3f3db66589c11f8ddacfeaecc03fb80c510 and has less performance
benefit.

The new layout algorithm also addresses the following problems:

* Size change of MCFillFragment/MCOrgFragment did not influence the
  fixed-point iteration, which could be problematic for contrived cases.
* The `invalid number of bytes` error was reported too early. Since
  `.zero A-B` might have temporary negative values in the first few
  iterations.
* X86AsmBackend::finishLayout performed only one iteration, which might
  not converge. In addition, the removed `#ifndef NDEBUG` code (disabled
  by default) in X86AsmBackend::finishLayout was problematic, as !NDEBUG
  and NDEBUG builds evaluated fragment offsets at different times before
  this patch.
* The computed layout for relax-recompute-align.s is optimal now.

Builds with many text sections (e.g. full LTO) shall observe a decrease
in compile time while the new algorithm could be slightly slower for
some -O0 -g projects.

Aligned bundling from the deprecated PNaCl placed constraints how we can
perform iteration.

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