Object/COFF: Add a struct for the function table in .pdata.

This is the data structure listed on Microsoft PE/COFF Spec Revision 8.3, p. 80.
The name of the struct is not mentioned in the Microsoft PE/COFF spec, so I made
it up.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202438 91177308-0d34-0410-b5e6-96231b3b80d8