[ELF] Correct sh_info for static symbol table

    
The sh_info field of the SHT_SYMTAB section holds the index for the
first non-local symbol. When there are global symbols that are output
with STB_LOCAL binding due to having hidden visibility or matching
the local version from a version script, the calculated value of
NumLocals + 1 does not account for them. This change accounts for
global symbols being output with local binding.

Differential Revision: https://reviews.llvm.org/D28950

llvm-svn: 292910