Re-submit r195852 with GroupedSectionsPass change.

GroupedSectionsPass was a complicated pass. That pass's job was to reorder
atoms by section name, so that the atoms with the same section prefix will be
emitted consecutively to the executable. The pass added layout edges to atoms,
and let the layout pass to actually reorder them.

This patch simplifies the design by making GroupedSectionPass to directly
reorder atoms, rather than adding layout edges. This resembles ELF's
ArrayOrderPass.

This patch improves the performance of LLD; it used to take 7.1 seconds to
link LLD with LLD on my Macbook Pro, but it now takes 6.1 seconds.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@196628 91177308-0d34-0410-b5e6-96231b3b80d8
Optimize the layout pass.

The comparator used in the layout pass has many calls of map::find(). Because
std::sort runs the comparator N*log2(N) times, the maps are looked up with the
same key again and again. The map lookup is not a very fast operation. It made
the pass slow.

This patch eliminates the duplicate map lookups using decorate-sort-undecorate
idiom. The pass used to take 1.1 seconds when linking LLD with LLD on Windows,
but it now takes only 0.3 seconds. Overall performance gain in that case is from
6.1 seconds to 5.2 seconds.

Differential Revision: http://llvm-reviews.chandlerc.com/D2358

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@196714 91177308-0d34-0410-b5e6-96231b3b80d8