[yaml2obj] - Check we correctly set the sh_info field of .symtab section.

initSymtabSectionHeader has the following line:
SHeader.sh_info = findLocalsNum(Symbols) + 1;

As was mentioned in a review comments for D60122,
it is never tested. The patch adds a test.

Differential revision: https://reviews.llvm.org/D60192

llvm-svn: 357687