Revert r203962 and two revisions depending on it: r204028 and r204059.
The revision I'm reverting breaks handling of transitive aliases. This blocks us
and breaks sanitizer bootstrap:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap/builds/2651
(and checked locally by Alexey).

This revision is the result of:

  svn merge -r204059:204058 -r204028:204027 -r203962:203961 .

+ the regression test added to test/MC/ELF/alias.s

Another way to reproduce the regression with clang:
  $ cat q.c
  void a1();
  void a2() __attribute__((alias("a1")));
  void a3() __attribute__((alias("a2")));
  void a1() {}

  $ ~/work/llvm-build/bin/clang-3.5-good -c q.c && mv q.o good.o && \
      ~/work/llvm-build/bin/clang-3.5-bad -c q.c && mv q.o bad.o && \
      objdump -t good.o bad.o

    good.o:     file format elf64-x86-64

    SYMBOL TABLE:
    0000000000000000 l    df *ABS*  0000000000000000 q.c
    0000000000000000 l    d  .text  0000000000000000 .text
    0000000000000000 l    d  .data  0000000000000000 .data
    0000000000000000 l    d  .bss   0000000000000000 .bss
    0000000000000000 l    d  .comment       0000000000000000 .comment
    0000000000000000 l    d  .note.GNU-stack        0000000000000000 .note.GNU-stack
    0000000000000000 l    d  .eh_frame      0000000000000000 .eh_frame
    0000000000000000 g     F .text  0000000000000006 a1
    0000000000000000 g     F .text  0000000000000006 a2
    0000000000000000 g     F .text  0000000000000006 a3



    bad.o:     file format elf64-x86-64

    SYMBOL TABLE:
    0000000000000000 l    df *ABS*  0000000000000000 q.c
    0000000000000000 l    d  .text  0000000000000000 .text
    0000000000000000 l    d  .data  0000000000000000 .data
    0000000000000000 l    d  .bss   0000000000000000 .bss
    0000000000000000 l    d  .comment       0000000000000000 .comment
    0000000000000000 l    d  .note.GNU-stack        0000000000000000 .note.GNU-stack
    0000000000000000 l    d  .eh_frame      0000000000000000 .eh_frame
    0000000000000000 g     F .text  0000000000000006 a1
    0000000000000000 g     F .text  0000000000000006 a2
    0000000000000000 g       .text  0000000000000000 a3