[PECOFF][Writer] Compute the size of a chunk each time it's added rather than all at once.

Contents of ".reloc" section depends on the addresses of other sections, so
the section cannot be created until all the other sections are created and get
their memory addresses (RVAs). That means that computation of section size
needs to be at least two pass.

Techynically there's no reason to compute it all at once, but instead we can
compute the address of a section as added to the output file. Doing so helps
us to create ".reloc" section.

llvm-svn: 185902