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>

test/BUILD.gn

@@ -229,10 +229,10 @@ tint_unittests_source_set("tint_unittests_core_src") {
     "../src/resolver/assignment_validation_test.cc",
     "../src/resolver/atomics_test.cc",
     "../src/resolver/atomics_validation_test.cc",
-    "../src/resolver/block_test.cc",
     "../src/resolver/builtins_validation_test.cc",
     "../src/resolver/call_test.cc",
     "../src/resolver/call_validation_test.cc",
+    "../src/resolver/compound_statement_test.cc",
     "../src/resolver/control_block_validation_test.cc",
     "../src/resolver/decoration_validation_test.cc",
     "../src/resolver/entry_point_validation_test.cc",