Make SectionPiece 8 bytes smaller on LP64.

This patch makes SectionPiece class 8 bytes smaller on platforms
on which pointer size is 8 bytes. Sean suggested in a post commit
review for r270340 that this could make a differentce, and it
actually is. Time to link clang (with debug info) improved from
6.725 seconds to 6.589 seconds or by about 2%.

Differential Revision: http://reviews.llvm.org/D20613

llvm-svn: 270717