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/buffer/uniform/static_index/read.wgsl.expected.fxc.hlsl

@@ -0,0 +1,57 @@
+struct Inner {
+  int x;
+};
+
+cbuffer cbuffer_s : register(b0, space0) {
+  uint4 s[13];
+};
+
+float2x3 tint_symbol_7(uint4 buffer[13], uint offset) {
+  const uint scalar_offset = ((offset + 0u)) / 4;
+  const uint scalar_offset_1 = ((offset + 16u)) / 4;
+  return float2x3(asfloat(buffer[scalar_offset / 4].xyz), asfloat(buffer[scalar_offset_1 / 4].xyz));
+}
+
+float3x2 tint_symbol_8(uint4 buffer[13], uint offset) {
+  const uint scalar_offset_2 = ((offset + 0u)) / 4;
+  uint4 ubo_load = buffer[scalar_offset_2 / 4];
+  const uint scalar_offset_3 = ((offset + 8u)) / 4;
+  uint4 ubo_load_1 = buffer[scalar_offset_3 / 4];
+  const uint scalar_offset_4 = ((offset + 16u)) / 4;
+  uint4 ubo_load_2 = buffer[scalar_offset_4 / 4];
+  return float3x2(asfloat(((scalar_offset_2 & 2) ? ubo_load.zw : ubo_load.xy)), asfloat(((scalar_offset_3 & 2) ? ubo_load_1.zw : ubo_load_1.xy)), asfloat(((scalar_offset_4 & 2) ? ubo_load_2.zw : ubo_load_2.xy)));
+}
+
+Inner tint_symbol_10(uint4 buffer[13], uint offset) {
+  const uint scalar_offset_5 = ((offset + 0u)) / 4;
+  const Inner tint_symbol_12 = {asint(buffer[scalar_offset_5 / 4][scalar_offset_5 % 4])};
+  return tint_symbol_12;
+}
+
+typedef Inner tint_symbol_11_ret[4];
+tint_symbol_11_ret tint_symbol_11(uint4 buffer[13], uint offset) {
+  Inner arr[4] = (Inner[4])0;
+  {
+    [loop] for(uint i_1 = 0u; (i_1 < 4u); i_1 = (i_1 + 1u)) {
+      arr[i_1] = tint_symbol_10(buffer, (offset + (i_1 * 16u)));
+    }
+  }
+  return arr;
+}
+
+[numthreads(1, 1, 1)]
+void main() {
+  const int3 a = asint(s[0].xyz);
+  const int b = asint(s[0].w);
+  const uint3 c = s[1].xyz;
+  const uint d = s[1].w;
+  const float3 e = asfloat(s[2].xyz);
+  const float f = asfloat(s[2].w);
+  const int2 g = asint(s[3].xy);
+  const int2 h = asint(s[3].zw);
+  const float2x3 i = tint_symbol_7(s, 64u);
+  const float3x2 j = tint_symbol_8(s, 96u);
+  const Inner k = tint_symbol_10(s, 128u);
+  const Inner l[4] = tint_symbol_11(s, 144u);
+  return;
+}