[Clang] Respect -L options when compiling directly for AMDGPU

The AMDGPU linker is `lld`, which has full support for standard features
like static libraries. Previously the AMDGPU toolchain did not forward
`-L` arguments so we could not tell it where to find certain libraries.
This patch simply forwards it like the other toolchains.

Reviewed By: yaxunl, MaskRay

Differential Revision: https://reviews.llvm.org/D150013

clang/lib/Driver/ToolChains/CommonArgs.cpp

@@ -232,9 +232,11 @@ void tools::AddLinkerInputs(const ToolChain &TC, const InputInfoList &Inputs,
   Args.AddAllArgValues(CmdArgs, options::OPT_Zlinker_input);
 
   // LIBRARY_PATH are included before user inputs and only supported on native
-  // toolchains.
+  // toolchains. Otherwise only add the '-L' arguments requested by the user.
   if (!TC.isCrossCompiling())
     addDirectoryList(Args, CmdArgs, "-L", "LIBRARY_PATH");
+  else
+    Args.AddAllArgs(CmdArgs, options::OPT_L);
 
   for (const auto &II : Inputs) {
     // If the current tool chain refers to an OpenMP offloading host, we

clang/test/Driver/amdgpu-toolchain.c

@@ -11,6 +11,6 @@
 // DWARF_VER: "-dwarf-version=5"
 
 // RUN: %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx906 -nogpulib \
-// RUN:   -flto -fconvergent-functions %s 2>&1 | FileCheck -check-prefix=LTO %s
+// RUN:   -L. -flto -fconvergent-functions %s 2>&1 | FileCheck -check-prefix=LTO %s
 // LTO: clang{{.*}} "-flto=full"{{.*}}"-fconvergent-functions"
-// LTO: ld.lld{{.*}}-plugin-opt=mcpu=gfx906
+// LTO: ld.lld{{.*}}"-L."{{.*}}"-plugin-opt=mcpu=gfx906"