Prefer to put STDOUT CHECK at the end of the file. (#3073)

Allow interleaving of STDOUT and STDERR check lines. Put STDOUT lines
after the line they're attached to, and STDERR lines before. If no
STDOUT check line is attached to any line, then put them all at the end
of the file instead.

This is intended to better handle the case where stdout contains
unreplaced mentions of line numbers, and also reflects that stdout is
typically a consequence of the test rather than commentary on it, so
placing it after the test seems likely to read better.

toolchain/parser/testdata/function/declaration/basic.carbon

@@ -3,6 +3,9 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
+
+fn F();
+
 // CHECK:STDOUT: [
 // CHECK:STDOUT:   {kind: 'FunctionIntroducer', text: 'fn'},
 // CHECK:STDOUT:   {kind: 'Name', text: 'F'},
@@ -11,5 +14,3 @@
 // CHECK:STDOUT: {kind: 'FunctionDeclaration', text: ';', subtree_size: 5},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]
-
-fn F();