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/struct/type_constructor.wgsl.expected.fxc.hlsl

@@ -0,0 +1,58 @@
+struct S1 {
+  int a;
+  int b;
+  int c;
+  int d;
+};
+struct S2 {
+  int e;
+  S1 f;
+};
+struct S3 {
+  int g;
+  S1 h;
+  S2 i;
+};
+struct T {
+  int a[2];
+};
+
+[numthreads(1, 1, 1)]
+void main() {
+  const int x = 42;
+  const S1 empty = (S1)0;
+  const S1 nonempty = {1, 2, 3, 4};
+  const S1 nonempty_with_expr = {1, x, (x + 1), nonempty.d};
+  const S3 nested_empty = (S3)0;
+  const S1 tint_symbol = {2, 3, 4, 5};
+  const S1 tint_symbol_1 = {7, 8, 9, 10};
+  const S2 tint_symbol_2 = {6, tint_symbol_1};
+  const S3 nested_nonempty = {1, tint_symbol, tint_symbol_2};
+  const S1 tint_symbol_3 = {2, x, (x + 1), nested_nonempty.i.f.d};
+  const S2 tint_symbol_4 = {6, nonempty};
+  const S3 nested_nonempty_with_expr = {1, tint_symbol_3, tint_symbol_4};
+  const S1 tint_symbol_5 = (S1)0;
+  const int subexpr_empty = tint_symbol_5.a;
+  const S1 tint_symbol_6 = {1, 2, 3, 4};
+  const int subexpr_nonempty = tint_symbol_6.b;
+  const S1 tint_symbol_7 = {1, x, (x + 1), nonempty.d};
+  const int subexpr_nonempty_with_expr = tint_symbol_7.c;
+  const S2 tint_symbol_8 = (S2)0;
+  const S1 subexpr_nested_empty = tint_symbol_8.f;
+  const S1 tint_symbol_9 = {2, 3, 4, 5};
+  const S2 tint_symbol_10 = {1, tint_symbol_9};
+  const S1 subexpr_nested_nonempty = tint_symbol_10.f;
+  const S1 tint_symbol_11 = {2, x, (x + 1), nested_nonempty.i.f.d};
+  const S2 tint_symbol_12 = {1, tint_symbol_11};
+  const S1 subexpr_nested_nonempty_with_expr = tint_symbol_12.f;
+  const T aosoa_empty[2] = (T[2])0;
+  const int tint_symbol_13[2] = {1, 2};
+  const T tint_symbol_14 = {tint_symbol_13};
+  const int tint_symbol_15[2] = {3, 4};
+  const T tint_symbol_16 = {tint_symbol_15};
+  const T aosoa_nonempty[2] = {tint_symbol_14, tint_symbol_16};
+  const int tint_symbol_17[2] = {1, (aosoa_nonempty[0].a[0] + 1)};
+  const T tint_symbol_18 = {tint_symbol_17};
+  const T aosoa_nonempty_with_expr[2] = {tint_symbol_18, aosoa_nonempty[1]};
+  return;
+}