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/expressions/splat/immediate/bool.wgsl.expected.fxc.hlsl

@@ -0,0 +1,10 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  bool2 v2 = (true).xx;
+  bool3 v3 = (true).xxx;
+  bool4 v4 = (true).xxxx;
+}

test/tint/expressions/splat/immediate/f32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,10 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  float2 v2 = (1.0f).xx;
+  float3 v3 = (1.0f).xxx;
+  float4 v4 = (1.0f).xxxx;
+}

test/tint/expressions/splat/immediate/i32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,10 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  int2 v2 = (1).xx;
+  int3 v3 = (1).xxx;
+  int4 v4 = (1).xxxx;
+}

test/tint/expressions/splat/immediate/u32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,10 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  uint2 v2 = (1u).xx;
+  uint3 v3 = (1u).xxx;
+  uint4 v4 = (1u).xxxx;
+}