[HIP-Clang, test] Fix use of undef FileCheck var

[thopre]

Summary

Commit 8129521318accc44c2a009647572f6ebd3fc56dd changed a line defining
PREFIX in clang test CodeGenCUDA/device-stub.cu into a CHECK-NOT
directive. All following lines using PREFIX are therefore using an
undefined variable since the pattern defining PREFIX is not supposed to
occur and CHECK-NOT are checked independently.

This commit replaces all uses of PREFIX by the regex used to define it,
thereby avoiding the problem.

[yaxunl]

LGTM. Thanks.

clang/test/CodeGenCUDA/device-stub.cu

 // HIP: exit:
 
 // There should be no __[[PREFIX]]_register_globals if we have no
 // device-side globals, but we still need to register GPU binary.
 // Skip GPU binary string first.
 // CUDANOGLOBALS-NOT: @{{.*}} = private constant{{.*}}
 // HIPNOGLOBALS-NOT: @{{.*}} = internal constant{{.*}}
 // NOGLOBALS-NOT: define internal void @__{{.*}}_register_globals
-// NOGLOBALS-NOT: define internal void @__[[PREFIX:cuda|hip]]_module_ctor
-// NOGLOBALS-NOT: call{{.*}}[[PREFIX]]RegisterFatBinary{{.*}}__[[PREFIX]]_fatbin_wrapper
-// NOGLOBALS-NOT: call void @__[[PREFIX]]_register_globals
-// NOGLOBALS-NOT: define internal void @__[[PREFIX]]_module_dtor
-// NOGLOBALS-NOT: call void @__[[PREFIX]]UnregisterFatBinary
+// NOGLOBALS-NOT: define internal void @__{{cuda|hip}}_module_ctor
+// NOGLOBALS-NOT: call{{.*}}{{cuda|hip}}RegisterFatBinary{{.*}}__{{cuda|hip}}_fatbin_wrapper
+// NOGLOBALS-NOT: call void @__{{cuda|hip}}_register_globals
+// NOGLOBALS-NOT: define internal void @__{{cuda|hip}}_module_dtor
+// NOGLOBALS-NOT: call void @__{{cuda|hip}}UnregisterFatBinary
 
 // There should be no constructors/destructors if we have no GPU binary.
-// NOGPUBIN-NOT: define internal void @__[[PREFIX]]_register_globals
-// NOGPUBIN-NOT: define internal void @__[[PREFIX]]_module_ctor
-// NOGPUBIN-NOT: define internal void @__[[PREFIX]]_module_dtor
+// NOGPUBIN-NOT: define internal void @__{{cuda|hip}}_register_globals
+// NOGPUBIN-NOT: define internal void @__{{cuda|hip}}_module_ctor
+// NOGPUBIN-NOT: define internal void @__{{cuda|hip}}_module_dtor