Object, COFF: Cap the section contents to min(VirtualSize, SizeOfRawData)

It is not useful to return the data beyond VirtualSize it's less than
SizeOfRawData.

An implementation detail of COFF requires the section size to be rounded
up to a multiple of FileAlignment; this means that SizeOfRawData is not
representative of how large the section is.  Instead, we should cap it
to VirtualSize when this occurs as it represents the true size of the
section.

Note that this is only relevant in executable files because this
rounding doesn't occur in object files (and VirtualSize is always zero).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219388 91177308-0d34-0410-b5e6-96231b3b80d8