[Clang][SYCL] Fix -march= corruption for NativeCPU when BoundArch is empty (#22432)
BoundArch("") sets Arch=OffloadArch::Unknown (not Unused), so operator
bool() returns true even for an empty arch name. This caused
TranslateArgs to erase -march=<value> and re-add -march= (empty),
producing "unsupported argument '' to option '-march='".
Fix: use !BA.empty() (checks ArchName string) instead of if (BA) (checks Arch != Unused).
Broken by 448b725bb78b.
Fixes test Driver/sycl-native-cpu.cpp
CMPLRLLVM-76405
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
(cherry picked from commit 2c120ab84a5bb6d6000db8d13c8692afdfb4bcec)