Add while statement parsing.

This CL adds parsing for the WGSL `while` statement.

Bug: tint:1425
Change-Id: Ibce5e28568935ca4f51b5ac33e7a60af7a916b4a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/93540
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>

docs/tint/compound_statements.md

@@ -58,6 +58,26 @@ sem::ForLoopStatement {
 }
 ```
 
+## while
+
+WGSL:
+```
+while (condition) {
+    statement;
+}
+```
+
+Semantic tree:
+```
+sem::WhileStatement {
+    sem::Expression condition
+
+    sem::LoopBlockStatement {
+        sem::Statement statement
+    }
+}
+```
+
 ## loop
 
 WGSL: