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/identifiers/underscore/prefix/upper/alias.wgsl.expected.fxc.hlsl

@@ -0,0 +1,9 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  int c = 0;
+  int d = 0;
+}

test/tint/identifiers/underscore/prefix/upper/fn.wgsl.expected.fxc.hlsl

@@ -0,0 +1,18 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void A() {
+}
+
+void _A() {
+}
+
+void B() {
+  A();
+}
+
+void _B() {
+  _A();
+}

test/tint/identifiers/underscore/prefix/upper/let.wgsl.expected.fxc.hlsl

@@ -0,0 +1,9 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  const int B = 1;
+  const int _B = 2;
+}

test/tint/identifiers/underscore/prefix/upper/parameter.wgsl.expected.fxc.hlsl

@@ -0,0 +1,8 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f(int _A) {
+  const int B = _A;
+}

test/tint/identifiers/underscore/prefix/upper/struct.wgsl.expected.fxc.hlsl

@@ -0,0 +1,13 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+struct _A {
+  int _B;
+};
+
+void f() {
+  const _A c = (_A)0;
+  const int d = c._B;
+}

test/tint/identifiers/underscore/prefix/upper/var.wgsl.expected.fxc.hlsl

@@ -0,0 +1,12 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+static int A = 1;
+static int _A = 2;
+
+void f() {
+  int B = A;
+  int _B = _A;
+}