writer/hlsl: Emit static for private global variables

Fixed: tint:812
Change-Id: I822eb6b13f07a74eb47c4c8e18d1f75dcc4818bf
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/51366
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: David Neto <dneto@google.com>

test/ptr_ref/store/global/i32.spvasm.expected.hlsl

@@ -1,4 +1,4 @@
-int I = 0;
+static int I = 0;
 
 [numthreads(1, 1, 1)]
 void main() {

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

@@ -1,4 +1,4 @@
-int I;
+static int I;
 
 [numthreads(1, 1, 1)]
 void main() {

test/ptr_ref/store/global/struct_field.spvasm.expected.hlsl

@@ -1 +1,12 @@
-SKIP: error: pointers not supported in HLSL
+struct S {
+  int i;
+};
+
+static S V;
+
+[numthreads(1, 1, 1)]
+void main() {
+  V.i = 5;
+  return;
+}
+

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

@@ -1 +1 @@
-SKIP: error: pointers not supported in HLSL
+SKIP: Failed to generate: error: pointers not supported in HLSL