[RuntimeDyld] Change the return type of decodeAddend to match the storage type.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213686 91177308-0d34-0410-b5e6-96231b3b80d8
[RuntimeDyld][MachO][AArch64] Add assertion to check for duplicate addend definition.

In MachO for AArch64 it is possible to have an explicit addend defined by
the ARM64_RELOC_ADDEND relocation or having an addend encoded within the
instruction. Only one of them are allowed per relocation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213687 91177308-0d34-0410-b5e6-96231b3b80d8
[RuntimeDyld][MachO][AArch64] Implement the decodeAddend method.

This adds the required functionality to decode the immediate encoded in an
instruction that is referenced in a relocation entry.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213688 91177308-0d34-0410-b5e6-96231b3b80d8
[RuntimeDyld][MachO][AArch64] Add a helper function for encoding addends in instructions.

Factor out the addend encoding into a helper function and simplify the
processRelocationRef.

Also add a few simple rtdyld tests. More tests to come once GOTs can be tested too.

Related to <rdar://problem/17768539>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213689 91177308-0d34-0410-b5e6-96231b3b80d8