[SYCL][AMDGCN] Provide a more helpful --offload-arch error (#13078)

If multiple SYCL targets are provided, asking users to specify
`-Xsycl-target-backend --offload-arch` will fail for another reason,
which isn't a nice user experience.

Instead, we can conditionally tweak the error message so it shows the
most concise invocation required for it to work every time. This
preserves the old form when only one SYCL target is used - mostly for
consistency - but provides the more verbose form when it's required.

Author: frasercrmck

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -391,7 +391,7 @@ def err_drv_fsycl_unsupported_with_opt
 def warn_drv_opt_requires_opt
   : Warning<"'%0' should be used only in conjunction with '%1'">, InGroup<UnusedCommandLineArgument>;
 def err_drv_sycl_missing_amdgpu_arch : Error<
-  "missing AMDGPU architecture for SYCL offloading; specify it with '-Xsycl-target-backend --offload-arch'">;
+  "missing AMDGPU architecture for SYCL offloading; specify it with '-Xsycl-target-backend%select{|=%1}0 --offload-arch=<arch-name>'">;
 def warn_drv_sycl_offload_target_duplicate : Warning<
   "SYCL offloading target '%0' is similar to target '%1' already specified; "
   "will be ignored">, InGroup<SyclTarget>;

clang/lib/Driver/Driver.cpp

@@ -6100,7 +6100,8 @@ class OffloadingActionBuilder final {
         if (Triple.isAMDGCN() && llvm::none_of(GpuArchList, [&](auto &P) {
               return P.first.isAMDGCN();
             })) {
-          C.getDriver().Diag(clang::diag::err_drv_sycl_missing_amdgpu_arch);
+          C.getDriver().Diag(clang::diag::err_drv_sycl_missing_amdgpu_arch)
+              << (SYCLTripleList.size() > 1) << Triple.str();
           return true;
         }
       }

clang/test/Driver/sycl-offload-amdgcn.cpp

@@ -7,7 +7,12 @@
 // RUN: not %clangxx -### -std=c++11 -target x86_64-unknown-linux-gnu -fsycl \
 // RUN: -fsycl-targets=amdgcn-amd-amdhsa %s 2>&1 \
 // RUN: | FileCheck -check-prefix=CHK-ARCH %s
-// CHK-ARCH: error: missing AMDGPU architecture for SYCL offloading; specify it with '-Xsycl-target-backend --offload-arch'
+// CHK-ARCH: error: missing AMDGPU architecture for SYCL offloading; specify it with '-Xsycl-target-backend --offload-arch=<arch-name>'
+
+// RUN: not %clangxx -### -std=c++11 -target x86_64-unknown-linux-gnu -fsycl \
+// RUN: -fsycl-targets=spir64,amdgcn-amd-amdhsa %s 2>&1 \
+// RUN: | FileCheck -check-prefix=CHK-MULTI-ARCH %s
+// CHK-MULTI-ARCH: error: missing AMDGPU architecture for SYCL offloading; specify it with '-Xsycl-target-backend=amdgcn-amd-amdhsa --offload-arch=<arch-name>'
 
 /// Check action graph.
 // RUN: %clangxx -### -std=c++11 -target x86_64-unknown-linux-gnu -fsycl \