[parse] Implement initial parsing support for Lambda expressions (#6583)

This adds the necessary parser infrastructure to recognize and parse
lambda expressions in Carbon.

Key changes:
- Added  and  Parse Node Kinds.
- Updated  to use  to accommodate the growing number of node kinds.
- Implemented parser states and handlers for lambda syntax ( or ).
- Added  structure to .
- Added diagnostics for missing lambda bodies.
- Added a stub in  phase to defer semantic analysis using .
- Added parser tests for lambdas.

toolchain/check/testdata/function/declaration/fail_todo_no_params.carbon

@@ -53,14 +53,14 @@ fn A[] -> ();
 
 // TODO: We don't have parsing support for this yet.
 library "[[@TEST_NAME]]";
-// CHECK:STDERR: fail_todo_arrow_body.carbon:[[@LINE+8]]:6: error: `fn` declarations must either end with a `;` or have a `{ ... }` block for a definition [ExpectedDeclSemiOrDefinition]
+// CHECK:STDERR: fail_todo_arrow_body.carbon:[[@LINE+8]]:1: error: semantics TODO: `function with positional parameters` [SemanticsTodo]
+// CHECK:STDERR: fn A => 0;
+// CHECK:STDERR: ^~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_todo_arrow_body.carbon:[[@LINE+4]]:6: error: semantics TODO: `HandleTerseBodyArrow` [SemanticsTodo]
 // CHECK:STDERR: fn A => 0;
 // CHECK:STDERR:      ^~
 // CHECK:STDERR:
-// CHECK:STDERR: fail_todo_arrow_body.carbon:[[@LINE+4]]:1: error: semantics TODO: `handle invalid parse trees in `check`` [SemanticsTodo]
-// CHECK:STDERR: fn A => 0;
-// CHECK:STDERR: ^~~~~~~~~~
-// CHECK:STDERR:
 fn A => 0;
 
 // --- fail_invalid_file_generic_regression_test.carbon
@@ -176,6 +176,11 @@ fn A {
 // CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_todo_arrow_body.carbon
 // CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @A;
+// CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_invalid_file_generic_regression_test.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {