[Driver][SYCL] Improve library resolution for libsycl (#6625)

When using -print-file-name=libsycl.so, we were only emitting the proper
location when -fsycl is being used.  Update this to allow for finding the
library just based on its existence in known locations.

Author: mdtoguchi

clang/lib/Driver/ToolChains/Linux.cpp

@@ -317,7 +317,8 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   // TODO Remove once LLVM_ENABLE_PROJECTS=libcxx is unsupported.
   if (StringRef(D.Dir).startswith(SysRoot) &&
       (D.getVFS().exists(D.Dir + "/../lib/libc++.so") ||
-       Args.hasArg(options::OPT_fsycl)))
+       Args.hasArg(options::OPT_fsycl) ||
+       D.getVFS().exists(D.Dir + "/../lib/libsycl.so")))
     addPathIfExists(D, D.Dir + "/../lib", Paths);
 
   addPathIfExists(D, concat(SysRoot, "/lib"), Paths);

clang/test/Driver/libsycl-print-file-name.c

@@ -0,0 +1,6 @@
+// UNSUPPORTED: system-windows
+
+// -print-file-name=libsycl.so
+// RUN: %clangxx -print-file-name=libsycl.so 2>&1 | FileCheck %s --check-prefix=PRINT_LIBSYCL
+// RUN: %clangxx -fsycl -print-file-name=libsycl.so 2>&1 | FileCheck %s --check-prefix=PRINT_LIBSYCL
+// PRINT_LIBSYCL: {{.*}}lib{{(\\\\|/)}}libsycl.so