Make the tree structure more explicit in parse dumps (#7591)

It's hard to track 2-space indent levels across large vertical gaps, and
it can be hard to suppress the instinct to read the dump as if it were
preorder. This change introduces a new dump mode that addresses both
problems by using box-drawing characters to explicitly represent the
parent-child edges of the tree. This new mode is the default, but the
old behavior remains available with
`--parse-dump-format=yaml-postorder`.

toolchain/parse/coverage_test.cpp

@@ -23,8 +23,8 @@ constexpr NodeKind UntestedNodeKinds[] = {NodeKind::Placeholder};
 
 // Looks for node kinds that aren't covered by a file_test.
 TEST(Coverage, NodeKind) {
-  Testing::TestKindCoverage(absl::GetFlag(FLAGS_testdata_manifest),
-                            R"(kind: '(\w+)')", llvm::ArrayRef(NodeKinds),
+  Testing::TestKindCoverage(absl::GetFlag(FLAGS_testdata_manifest), R"(─(\w+))",
+                            llvm::ArrayRef(NodeKinds),
                             llvm::ArrayRef(UntestedNodeKinds));
 }