Ensure dxil validator 1.8 runs for launch type tests (#6209)

Internal pipeline issues revealed that certain launch type tests that
were recently merged were running with older validators.
This PR ensures that the tests run with validator version 1.8.

tools/clang/test/HLSLFileCheck/dxil/intrinsics/launch_types_coalescing.ll

@@ -1,4 +1,4 @@
-; RUN: %dxv %s | FileCheck %s
+; RUN: %dxilver 1.8 | %dxv %s | FileCheck %s
 
 target datalayout = "e-m:e-p:32:32-i1:32-i8:32-i16:32-i32:32-i64:64-f16:32-f32:32-f64:64-n8:16:32:64"
 target triple = "dxil-ms-dx"

tools/clang/test/HLSLFileCheck/dxil/intrinsics/launch_types_thread.ll

@@ -1,4 +1,4 @@
-; RUN: %dxv %s | FileCheck %s
+; RUN: %dxilver 1.8 | %dxv %s | FileCheck %s
 
 target datalayout = "e-m:e-p:32:32-i1:32-i8:32-i16:32-i32:32-i64:64-f16:32-f32:32-f64:64-n8:16:32:64"
 target triple = "dxil-ms-dx"