Support/COFF: Fix PEHeader struct, and define PE32Header as its alias.

This change does not affect anything because everybody seems to be using
Object/COFF.h instead. But the definition is not for PE32 but for PE32+,
so fix it anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200038 91177308-0d34-0410-b5e6-96231b3b80d8