Re-land r337333, "Teach Clang to emit address-significance tables.",

Re-land r337333, "Teach Clang to emit address-significance tables.",
which was reverted in r337336.

The problem that required a revert was fixed in r337338.

Also added a missing "REQUIRES: x86-registered-target" to one of
the tests.

Original commit message:
> Teach Clang to emit address-significance tables.
>
> By default, we emit an address-significance table on all ELF
> targets when the integrated assembler is enabled. The emission of an
> address-significance table can be controlled with the -faddrsig and
> -fno-addrsig flags.
>
> Differential Revision: https://reviews.llvm.org/D48155

llvm-svn: 337339