sem: Add CompoundStatement

This change introduces sem::CompoundStatement, a new base class for
statements that can hold other statements.

sem::BlockStatements now derives from sem::CompoundStatement, and
this change introduces the following new CompoundStatements:
* `sem::IfStatement`
* `sem::ElseStatement`
* `sem::ForLoopStatement`
* `sem::LoopStatement`
* `sem::SwitchStatement`.
These new CompoundStatements are now inserted into the semantic
tree as now documented in `docs/compound_statements.md`.

The `sem::BlockStatement::FindFirstParent()` methods have been
moved down to `sem::Statement`.

The `Resolver::BlockScope()` method has been replaced with
`Resolver::Scope()` which now maintains the `current_statement_`,
`current_compound_statement_ ` and `current_block_`. This
simplifies statement nesting.

The most significant change in behavior is that statements now
always have a parent, so calling Block() on the initializer or
continuing of a for-loop statement will now return the
BlockStatement that holds the for-loop. Before this would
return nullptr.

Fixed: tint:979
Change-Id: I90e38fd719da2a281ed9210e975ab96171cb6842
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/57707
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>

docs/compound_statements.md

@@ -0,0 +1,115 @@
+# Compound Statements
+
+Compound statements are statements that can hold other statements.
+
+This document maps the WGSL compound statements to their semantic tree representations.
+
+## if statement
+
+WGSL:
+```
+if (condition_a) {
+    statement_a;
+} else if (condition_b) {
+    statement_b;
+} else {
+    statement_c;
+}
+```
+
+Semantic tree:
+```
+sem::IfStatement {
+    condition_a
+    sem::BlockStatement {
+        statement_a
+    }
+    sem::ElseStatement {
+        condition_b
+        sem::BlockStatement {
+            statement_b
+        }
+    }
+    sem::ElseStatement {
+        sem::BlockStatement {
+            statement_c
+        }
+    }
+}
+```
+
+## for loop
+
+WGSL:
+```
+for (initializer; condition; continuing) {
+    statement;
+}
+```
+
+Semantic tree:
+```
+sem::ForLoopStatement {
+    sem::Statement  initializer
+    sem::Expression condition
+    sem::Statement  continuing
+
+    sem::LoopBlockStatement {
+        sem::Statement statement
+    }
+}
+```
+
+## loop
+
+WGSL:
+```
+loop (condition) {
+    statement_a;
+    continuing {
+        statement_b;
+    }
+}
+```
+
+Semantic tree:
+```
+sem::LoopStatement {
+    sem::Expression condition
+
+    sem::LoopBlockStatement {
+        sem::Statement statement_a
+        sem::LoopContinuingBlockStatement {
+            sem::Statement statement_b
+        }
+    }
+}
+```
+
+
+## switch statement
+
+WGSL:
+```
+switch (condition) {
+    case literal_a, literal_b: {
+        statement_a;
+    }
+    default {
+        statement_b;
+    }
+}
+```
+
+Semantic tree:
+```
+sem::SwitchStatement {
+    sem::Expression condition
+    sem::SwitchCaseBlockStatement {
+        sem::Statement statement_a
+    }
+    sem::SwitchCaseBlockStatement {
+        sem::Statement statement_b
+    }
+}
+```