[SYCL] Gracefully handle unknown device (#11254)

Don't throw ICE when an unknown device is specified explicitly via
`-Xsycl-target-backend --offload-arch=`. We don't enable macros or other
niceties from sycl_ext_oneapi_device_architecture, but at least the code
compiles.

Fixes #8112, #11203, #12010

Author: al42and

clang/lib/Driver/ToolChains/Clang.cpp

@@ -5482,7 +5482,7 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
            Triple.getSubArch() == llvm::Triple::SPIRSubArch_gen) ||
           Triple.isNVPTX() || Triple.isAMDGCN()) {
         StringRef Device = JA.getOffloadingArch();
-        if (!Device.empty()) {
+        if (!Device.empty() && !SYCL::gen::getGenDeviceMacro(Device).empty()) {
           Macro = "-D";
           Macro += SYCL::gen::getGenDeviceMacro(Device);
         }

clang/test/Driver/sycl-unsupported-arch.cpp

@@ -0,0 +1,8 @@
+/// Verify that compiler passes are correctly determined
+// RUN: %clangxx -fsycl -fsycl-targets=amdgcn-amd-amdhsa -Xsycl-target-backend=amdgcn-amd-amdhsa --offload-arch=gfx600 -nogpulib -c -ccc-print-phases %s 2>&1 | FileCheck %s --check-prefix=CHECK-PHASES
+// CHECK-PHASES: offload, "device-sycl (amdgcn-amd-amdhsa:gfx600)"
+
+/// Verify that preprocessor works (#8112)
+// RUN: %clangxx -fsycl -fsycl-targets=amdgcn-amd-amdhsa -Xsycl-target-backend=amdgcn-amd-amdhsa --offload-arch=gfx600 -nogpulib -E %s 2>&1 | FileCheck %s --check-prefix=CHECK-PREPROCESSOR
+// CHECK-PREPROCESSOR: // __CLANG_OFFLOAD_BUNDLE____START__ sycl-amdgcn-amd-amdhsa-gfx600
+