Allow LSP test_file to autofill didOpen params from previous splits (#5078)

Currently file tests for LSP must provide carbon source code as an
escaped string within notify params, i.e.
```
[[@LSP-NOTIFY:textDocument/didOpen:
  "textDocument": {
    "uri": "file:/class.carbon",
    "languageId": "carbon",
    "text": "class A {\n  fn F();\n  fn G() {}\n}\n"
  }
]]
```

This works fine for simple, single line files but gets annoying when
working with more complicated files which are necessary when testing
more complicated features e.g. goto-definition

```
--- class.carbon
class A {
  fn F();
  fn G() {}
}
--- STDIN
[[@LSP-NOTIFY:textDocument/didOpen:
  "textDocument": {
    "uri": "file:/class.carbon",
    "languageId": "carbon",
    "text": "AUTOFILL"
  }
]]
```

This PR extends file_test_base to be able to parse the notify/call
params and inject files from the test_file's splits into the JSON input.
I purposely avoid using the clangd types and manually parse the
llvm::json::Value here to avoid introducing a depdendency on clangd to
the generic file_test_base, but happy to change if we think that is
fine. Also happy to accept other suggestions on alternative methods to
achieve same result.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

testing/file_test/testdata/lsp_autofill.carbon

@@ -0,0 +1,38 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //testing/file_test:file_test_base_test --test_arg=--file_tests=testing/file_test/testdata/lsp_autofill.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //testing/file_test:file_test_base_test -- --dump_output --file_tests=testing/file_test/testdata/lsp_autofill.carbon
+
+// --- foo.carbon
+class Foo {
+  fn foo();
+  fn bar() {}
+}
+
+// --- STDIN
+[[@LSP-NOTIFY:textDocument/didOpen:
+  "textDocument": {
+    "uri": "file:/foo.carbon",
+    "languageId": "carbon",
+    "text": "FROM_FILE_SPLIT"
+  }
+]]
+
+// --- AUTOUPDATE-SPLIT
+
+// CHECK:STDERR: --- STDIN:
+// CHECK:STDERR: Content-Length: 182
+// CHECK:STDERR:
+// CHECK:STDERR: {"jsonrpc":"2.0","method":"textDocument/didOpen","params":{"textDocument":{"languageId":"carbon","text":"class Foo {\n  fn foo();\n  fn bar() {}\n}\n\n","uri":"file:/foo.carbon"}}}
+// CHECK:STDERR:
+// CHECK:STDERR:
+// CHECK:STDOUT: 2 args: `default_args`, `foo.carbon`
+// CHECK:STDOUT: foo.carbon:1: class Foo {
+// CHECK:STDOUT: foo.carbon:2:   fn foo();
+// CHECK:STDOUT: foo.carbon:3:   fn bar() {}
+// CHECK:STDOUT: foo.carbon:4: }

testing/file_test/testdata/lsp_keywords.carbon

@@ -11,62 +11,57 @@
 // --- STDIN
 [[@LSP:foo:]]
 [[@LSP:foo]]
-[[@LSP:bar:content]]
+[[@LSP:bar:"content": 0]]
 [[@LSP:baz:
-multi
-line
+"multi": 0,
+"line": 1
 ]]
-[[@LSP-CALL:bar:content]]
+[[@LSP-CALL:bar:"content": 0]]
 [[@LSP-CALL:baz:
-multi
-line]]
+"multi": 0,
+"line": 1]]
 [[@LSP-REPLY:7]]
-[[@LSP-REPLY:8:bar]]
+[[@LSP-REPLY:8:"bar": 0]]
 [[@LSP-NOTIFY:exit]]
 
 // --- AUTOUPDATE-SPLIT
 
 // CHECK:STDERR: --- STDIN:
-// CHECK:STDERR: Content-Length: 37
+// CHECK:STDERR: Content-Length: 34
 // CHECK:STDERR:
-// CHECK:STDERR: {"jsonrpc": "2.0", "method": "foo"}
+// CHECK:STDERR: {"jsonrpc":"2.0","method":"foo"}
 // CHECK:STDERR:
-// CHECK:STDERR: Content-Length: 37
+// CHECK:STDERR: Content-Length: 34
 // CHECK:STDERR:
-// CHECK:STDERR: {"jsonrpc": "2.0", "method": "foo"}
+// CHECK:STDERR: {"jsonrpc":"2.0","method":"foo"}
 // CHECK:STDERR:
 // CHECK:STDERR: Content-Length: 46
 // CHECK:STDERR:
-// CHECK:STDERR: {"jsonrpc": "2.0", "method": "bar", content}
+// CHECK:STDERR: {"jsonrpc":"2.0","method":"bar","content":0}
 // CHECK:STDERR:
-// CHECK:STDERR: Content-Length: 50
+// CHECK:STDERR: Content-Length: 53
 // CHECK:STDERR:
-// CHECK:STDERR: {"jsonrpc": "2.0", "method": "baz",
-// CHECK:STDERR: multi
-// CHECK:STDERR: line
-// CHECK:STDERR: }
+// CHECK:STDERR: {"jsonrpc":"2.0","method":"baz","multi":0,"line":1}
 // CHECK:STDERR:
-// CHECK:STDERR: Content-Length: 67
+// CHECK:STDERR: Content-Length: 64
 // CHECK:STDERR:
-// CHECK:STDERR: {"jsonrpc": "2.0", "method": "bar", "id": 1, "params": {content}}
+// CHECK:STDERR: {"jsonrpc":"2.0","method":"bar","id":1,"params":{"content":0}}
 // CHECK:STDERR:
 // CHECK:STDERR: Content-Length: 71
 // CHECK:STDERR:
-// CHECK:STDERR: {"jsonrpc": "2.0", "method": "baz", "id": 2, "params": {
-// CHECK:STDERR: multi
-// CHECK:STDERR: line}}
+// CHECK:STDERR: {"jsonrpc":"2.0","method":"baz","id":2,"params":{"line":1,"multi":0}}
 // CHECK:STDERR:
-// CHECK:STDERR: Content-Length: 31
+// CHECK:STDERR: Content-Length: 28
 // CHECK:STDERR:
-// CHECK:STDERR: {"jsonrpc": "2.0", "id": "7"}
+// CHECK:STDERR: {"jsonrpc":"2.0","id":"7"}
 // CHECK:STDERR:
-// CHECK:STDERR: Content-Length: 48
+// CHECK:STDERR: Content-Length: 47
 // CHECK:STDERR:
-// CHECK:STDERR: {"jsonrpc": "2.0", "id": "8", "result": {bar}}
+// CHECK:STDERR: {"jsonrpc":"2.0","id":"8","result":{"bar":0}}
 // CHECK:STDERR:
-// CHECK:STDERR: Content-Length: 38
+// CHECK:STDERR: Content-Length: 35
 // CHECK:STDERR:
-// CHECK:STDERR: {"jsonrpc": "2.0", "method": "exit"}
+// CHECK:STDERR: {"jsonrpc":"2.0","method":"exit"}
 // CHECK:STDERR:
 // CHECK:STDERR:
 // CHECK:STDOUT: 1 args: `default_args`