Refactor EHOutputSection.

This patch refactors EHOutputSection using SectionPiece struct.
EHRegion class was removed since we can now directly use SectionPiece.

An incomplete support of large CIE/FDE record (> 2^32 bytes) was removed
because it silently created broken executable. There are several places
in the existing code that "size" field is always 4 bytes and at offset 4
in the record, which is not true for 64-bit size records. We will have to
support that in future, but it is better to error out instead of creating
malformed eh_frame sections.

llvm-svn: 270382