Represent symbols with a SymbolIndex,SectionIndex pair.

With this patch we use the SectionIndex directly, instead of counting the
number of symbol tables. This saves a DenseMap lookup every time we want to
find which symbol a relocation refers to.

Also simplify based on the fact that there is at most one SHT_SYMTAB and one
SHT_DYNSYM.

llvm-svn: 183326