[Driver] Report warnings for unclaimed TargetSpecific options for assembler input

This patch amends D151590 to not error for unlaimed TargetSpecific
options for `-x assembler` input files. This input type causes Driver to
construct tools::ClangAs (-fintegrated-as) or other assemblers (e.g.
tools::gnutools::Assembler) Their ConstructJobs methods, unlike
Clang::ConstructJobs, claim very few options. If an option is unclaimed,
it either leads to a -Wunused-command-line-argument warning or an error
(if `TargetSpecific` is set):
```
% clang '-###' --target=aarch64 -mbranch-protection=bti -c a.s
clang: error: unsupported option '-mbranch-protection=' for target 'aarch64'
```

It seems that downgrading the diagnostic to warning is most useful as
many users use CFLAGS even for `.s` files:
```
clang --target=aarch64 -mbranch-protection=bti -S a.c
clang --target=aarch64 -mbranch-protection=bti -c a.s
```

I decide not to suppress the warning so that
-Wunused-command-line-argument lovers still get a warning, and help
projects use proper ASFLAGS/CFLAGS/etc.

Note: `-mbranch-protection=bti a.S` currently has no warning as `-x assembler-with-cpp`
instructs clangDriver to select tools::Clang and claim most options.

Revert D159010 to demonstrate that we emit a warning for -mfpmath= for
`-x assembler` input.

Modify my AIX cleanup cd18efb6 to
add an err_drv_unsupported_opt_for_target.

Reviewed By: thesamesam

Differential Revision: https://reviews.llvm.org/D159173