Rename P -> Pieces.
Rename P -> Pieces.

Conventionally, an array of SectionPieces is named Pieces.
It is better to follow the convention.

git-svn-id: f6089bf0e6284f307027cef4f64114ee9ebb0424@315543 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/lld,llvm-mirror/lld