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/statements/assign/phony/addr_of_runtime_array.wgsl.expected.fxc.hlsl

@@ -0,0 +1,6 @@
+RWByteAddressBuffer s : register(u0, space0);
+
+[numthreads(1, 1, 1)]
+void main() {
+  return;
+}

test/tint/statements/assign/phony/call.wgsl.expected.fxc.hlsl

@@ -0,0 +1,9 @@
+int f(int a, int b, int c) {
+  return ((a * b) + c);
+}
+
+[numthreads(1, 1, 1)]
+void main() {
+  f(1, 2, 3);
+  return;
+}

test/tint/statements/assign/phony/multiple_side_effects.wgsl.expected.fxc.hlsl

@@ -0,0 +1,12 @@
+int f(int a, int b, int c) {
+  return ((a * b) + c);
+}
+
+[numthreads(1, 1, 1)]
+void main() {
+  const int tint_symbol = f(1, 2, 3);
+  const int tint_symbol_1 = f(4, 5, 6);
+  const int tint_symbol_2 = f(8, 9, 10);
+  const int tint_symbol_3 = f(7, tint_symbol_2, 11);
+  return;
+}

test/tint/statements/assign/phony/storage_buffer.wgsl.expected.dxc.hlsl

@@ -0,0 +1,6 @@
+RWByteAddressBuffer s : register(u0, space0);
+
+[numthreads(1, 1, 1)]
+void main() {
+  return;
+}

test/tint/statements/assign/phony/storage_buffer.wgsl.expected.fxc.hlsl

@@ -0,0 +1,6 @@
+RWByteAddressBuffer s : register(u0, space0);
+
+[numthreads(1, 1, 1)]
+void main() {
+  return;
+}

test/tint/statements/assign/phony/uniform_buffer.wgsl.expected.fxc.hlsl

@@ -0,0 +1,8 @@
+cbuffer cbuffer_u : register(b0, space0) {
+  uint4 u[1];
+};
+
+[numthreads(1, 1, 1)]
+void main() {
+  return;
+}