hlsl: Pointer support

Add `transform::InlinePointerLets` - a Transform that moves all usage of function-scope  `let` statements of a pointer type into their places of usage.

Make the HLSL writer transform pointer parameters to `inout`.

Fixed: tint:183
Change-Id: I0a7552fa6cd31c7b7691e64feae3170a81cc6c49
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51281
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>

test/ptr_ref/store/local/i32.wgsl.expected.hlsl

@@ -1 +1,8 @@
-SKIP: Failed to generate: error: pointers not supported in HLSL
+[numthreads(1, 1, 1)]
+void main() {
+  int i = 123;
+  i = 123;
+  i = ((100 + 20) + 3);
+  return;
+}
+

test/ptr_ref/store/param/ptr.spvasm.expected.hlsl

@@ -1 +1,13 @@
-SKIP: error: pointers not supported in HLSL
+void func(int value, inout int pointer) {
+  pointer = value;
+  return;
+}
+
+[numthreads(1, 1, 1)]
+void main() {
+  int i = 0;
+  i = 123;
+  func(123, i);
+  return;
+}
+

test/ptr_ref/store/param/ptr.wgsl.expected.hlsl

@@ -1 +1,11 @@
-SKIP: error: pointers not supported in HLSL
+void func(int value, inout int pointer) {
+  pointer = value;
+}
+
+[numthreads(1, 1, 1)]
+void main() {
+  int i = 123;
+  func(123, i);
+  return;
+}
+