[PECOFF] Rename getSectionAlignment -> getSectionDefaultAlignment.

These fields are for /align option. Section alignment can be set per-section
basis with /section option too. In order to avoid name conflicts, rename the
existing identifiers to become more specific. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@194160 91177308-0d34-0410-b5e6-96231b3b80d8