tint/test-runner: Split expectations for FXC and DXC

Change tint's `--fxc` flag to take the path of the FXC compiler DLL.
Have tint attempt to validate with both FXC and DXC if `--validate` is
passed.

Fix the 'dirsWithNoPassExpectations' logic which looks like it got
broken with the tint -> dawn merge. It also incorrectly applied
filepath.FromSlash() on windows.

Change-Id: I0f46aa5c21bc48a2abc48402c41f846aff4a8633
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96800
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>

test/tint/builtins/textureNumSamples/depth_ms.spvasm.expected.fxc.hlsl

@@ -0,0 +1,63 @@
+Texture2DMS<float4> arg_0 : register(t0, space1);
+static float4 tint_symbol_1 = (0.0f).xxxx;
+
+void textureNumSamples_a3c8a0() {
+  int res = 0;
+  int3 tint_tmp;
+  arg_0.GetDimensions(tint_tmp.x, tint_tmp.y, tint_tmp.z);
+  const int x_16 = tint_tmp.z;
+  res = x_16;
+  return;
+}
+
+void tint_symbol_2(float4 tint_symbol) {
+  tint_symbol_1 = tint_symbol;
+  return;
+}
+
+void vertex_main_1() {
+  textureNumSamples_a3c8a0();
+  tint_symbol_2((0.0f).xxxx);
+  return;
+}
+
+struct vertex_main_out {
+  float4 tint_symbol_1_1;
+};
+struct tint_symbol_3 {
+  float4 tint_symbol_1_1 : SV_Position;
+};
+
+vertex_main_out vertex_main_inner() {
+  vertex_main_1();
+  const vertex_main_out tint_symbol_4 = {tint_symbol_1};
+  return tint_symbol_4;
+}
+
+tint_symbol_3 vertex_main() {
+  const vertex_main_out inner_result = vertex_main_inner();
+  tint_symbol_3 wrapper_result = (tint_symbol_3)0;
+  wrapper_result.tint_symbol_1_1 = inner_result.tint_symbol_1_1;
+  return wrapper_result;
+}
+
+void fragment_main_1() {
+  textureNumSamples_a3c8a0();
+  return;
+}
+
+void fragment_main() {
+  fragment_main_1();
+  return;
+}
+
+void compute_main_1() {
+  textureNumSamples_a3c8a0();
+  return;
+}
+
+[numthreads(1, 1, 1)]
+void compute_main() {
+  compute_main_1();
+  return;
+}