Underline the entire token when producing diagnostic messages. (#3413)

This is an incremental improvement on our diagnostic messages that
simply underlines an entire token if the token is larger than 1 char
(else it points to the single char with a caret like it used to).

toolchain/parse/testdata/function/declaration/fail_identifier_instead_of_sig.carbon

@@ -6,7 +6,7 @@
 
 // CHECK:STDERR: fail_identifier_instead_of_sig.carbon:[[@LINE+3]]:8: ERROR: `fn` requires a `(` for parameters.
 // CHECK:STDERR: fn foo bar;
-// CHECK:STDERR:        ^
+// CHECK:STDERR:        ^~~
 fn foo bar;
 
 // CHECK:STDOUT: - filename: fail_identifier_instead_of_sig.carbon

toolchain/parse/testdata/function/declaration/fail_repeated_fn_and_semi.carbon

@@ -6,7 +6,7 @@
 
 // CHECK:STDERR: fail_repeated_fn_and_semi.carbon:[[@LINE+3]]:4: ERROR: `fn` introducer should be followed by a name.
 // CHECK:STDERR: fn fn;
-// CHECK:STDERR:    ^
+// CHECK:STDERR:    ^~
 fn fn;
 
 // CHECK:STDOUT: - filename: fail_repeated_fn_and_semi.carbon

toolchain/parse/testdata/function/declaration/fail_skip_without_semi_to_curly.carbon

@@ -6,7 +6,7 @@
 
 // CHECK:STDERR: fail_skip_without_semi_to_curly.carbon:[[@LINE+3]]:1: ERROR: Unrecognized declaration introducer.
 // CHECK:STDERR: struct X { fn () }
-// CHECK:STDERR: ^
+// CHECK:STDERR: ^~~~~~
 struct X { fn () }
 fn F();