obj2yaml, COFF: Handle long section names

Long section names are represented as a slash followed by a numeric
ASCII string.  This number is an offset into a string table.

Print the appropriate entry in the string table instead of the less
enlightening /4.

N.B.  yaml2obj already does the right thing, this test exercises both
sides of the (de-)serialization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219458 91177308-0d34-0410-b5e6-96231b3b80d8