[Flang][Driver][Offload] Support -Xoffload-linker argument in Flang (llvm#109907)

The -Xoffload-linker command allows forwarding of linker commands to the
clang-linker-wrapper used for linking offload libraries into the
resulting offload binaries amongst other tasks. This is a rather useful
command to have to support the offloading programming models flang-new
currently aims to support (OpenMP/OpenACC).

Currently this flag is utilised in the check-offload tests after a
recent addition and is used in conjunction with the Fortran OpenMP test
suite there, which fails at the moment due to flang-new not recognizing
the command, this fixes the issue. The alternative to this would of
course be to setup the test config to avoid using this flag with
Fortran, but I believe adding support of the flag to flang-new has more
merit as having the same compatability/communication capabilities as
Clang to the clang-linker-wrapper is important as it's a critical
component of the offload pipeline, and the command will likely see more
use in the near future.

Author: agozillon

clang/include/clang/Driver/Options.td

@@ -1055,6 +1055,7 @@ def Xlinker : Separate<["-"], "Xlinker">, Flags<[LinkerInput, RenderAsInput]>,
   HelpText<"Pass <arg> to the linker">, MetaVarName<"<arg>">,
   Group<Link_Group>;
 def Xoffload_linker : JoinedAndSeparate<["-"], "Xoffload-linker">,
+  Visibility<[ClangOption, CLOption, FlangOption, DXCOption]>,
   HelpText<"Pass <arg> to the offload linkers or the ones identified by -<triple>">,
   MetaVarName<"<triple> <arg>">, Group<Link_Group>;
 def Xpreprocessor : Separate<["-"], "Xpreprocessor">, Group<Preprocessor_Group>,

flang/test/Driver/xoffload-linker.f90

@@ -0,0 +1,10 @@
+! Test the -Xoffload-linker flag that forwards link commands to the clang-linker-wrapper used
+! to help link offloading device libraries
+
+! RUN: %flang -### %s -o %t 2>&1 -fopenmp --offload-arch=gfx90a --target=aarch64-unknown-linux-gnu -nogpulib -Xoffload-linker a | FileCheck %s --check-prefix=CHECK-XLINKER
+
+! CHECK-XLINKER: "{{[^"]*}}clang-linker-wrapper{{.*}}"{{.*}}"--device-linker=a"{{.*}}
+
+! RUN: %flang -### %s -o %t 2>&1 -fopenmp --offload-arch=gfx90a --target=aarch64-unknown-linux-gnu -nogpulib -Xoffload-linker a -Xoffload-linker-amdgcn-amd-amdhsa b | FileCheck %s --check-prefix=CHECK-XLINKER-AMDGCN
+
+! CHECK-XLINKER-AMDGCN: "{{[^"]*}}clang-linker-wrapper{{.*}}"{{.*}}"--device-linker=a"{{.*}}"--device-linker=amdgcn-amd-amdhsa=b"{{.*}}