[Support] Add LLVM_IS_UNALIGNED_ACCESS_FAST.
[Object][Archive] Improve performance.

Improve performance of iterating over children and accessing the member file
buffer by caching the file size and moving code out to the header.

This also makes getBuffer return a StringRef instead of a MemoryBuffer. Both
fixing a memory leak and removing a malloc.

This takes getBuffer from ~10% of the time in lld to unmeasurable.
[Archive] Update to LLVM change.
[ELF] Fix uninitialized variable.
[ELF][Reader] Improve performance by removing calls to malloc.
[ELF] Use LLVM_IS_UNALIGNED_ACCESS_FAST.
[Resolver] Improve undefined symbol error.