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/shadowing/function/let.wgsl.expected.fxc.hlsl

@@ -0,0 +1,12 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void a() {
+  {
+    int a_1 = 1;
+    int b = a_1;
+  }
+  const int b = 1;
+}

test/tint/shadowing/function/param.wgsl.expected.fxc.hlsl

@@ -0,0 +1,8 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void a(int a_1) {
+  const int b = a_1;
+}

test/tint/shadowing/function/var.wgsl.expected.fxc.hlsl

@@ -0,0 +1,17 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+struct a {
+  int a;
+};
+
+void f() {
+  {
+    a a_1 = (a)0;
+    a b = a_1;
+  }
+  a a_2 = (a)0;
+  a b = a_2;
+}