Teach Clang to emit address-significance tables.

[pcc]

Summary

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.

Depends on D48143

[pcc]

• Add some additional driver tests

[rsmith]

Please add documentation and release notes for this flag too.