Optimize COFFObjectFile::sectionContainsSymbol a bit.

There is no need to compute the coff_section of the symbol just to compare the
pointer.

Inspired by the ELF implementation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219233 91177308-0d34-0410-b5e6-96231b3b80d8