[ELF] Regroup code creates ELF relocations references ELFReference into
a couple of new virtual functions.

Follow-up to the rL203408. Two virtual functions `createRelocationReference()`
responsible for creation of `ELFReference` have been replaced by a couple of
new virtual functions `createRelocationReferences()` (plural). Each former
function creates a //single// ELFReference for a specified `Elf_Rela`
or `Elf_Rel` relocation records. The new functions responsible for creation
of //all// relocation references for provided symbol.

For all targets except MIPS there are no functional changes.

MIPS ABI has a notion of //paired// relocations. An effective addend of such
relocations are calculated using addends of both pair's members.
Each `R_MIPS_HI16` and `R_MIPS_GOT16` (for local symbols) relocations must have
an associated `R_MIPS_LO16` entry immediately following it in the list
of relocations. Immediately does not mean "next record" in relocations section
but "next record referenced the same symbol". Moreover a single `R_MIPS_LO16`
relocation can be paired with multiple preceding `R_MIPS_HI16/R_MIPS_GOT16`
relocations.

The paired relocation can have offsets belong to the different symbols.
That is why we need to have access to list of all relocations during
construction of `ELFReference` for MIPS target.

The patch reviewed by Shankar Easwaran.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@206102 91177308-0d34-0410-b5e6-96231b3b80d8