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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183326 91177308-0d34-0410-b5e6-96231b3b80d8