PECOFF: Fix a "memset clearing an object of non-trivial type" warning

This time, the warning pointed to an actual problem, because the
coff_opt_header structure contained a std::vector. I guess this happened
to work because the all-zero state was a valid representation of an
empty vector, but its not a good idea to rely on that.

I remove the memset, and have the structure clear its members in the
constructor instead.

llvm-svn: 367299