Add support for testing with stdin (#4819)

In order to write language-server tests, we need some way to pass stdin
input. This adds support for a split "// --- STDIN" which will be
provided as a temp file for testing.

Note this does more stdin -> input_stream style renaming, this is just
bugging me more since I know shadowing works but it can be subtle to
read, particularly since I'm now making direct use of stdin in a handful
of spots.

toolchain/testing/file_test.cpp

@@ -35,7 +35,8 @@ class ToolchainFileTest : public FileTestBase {
 
   auto Run(const llvm::SmallVector<llvm::StringRef>& test_args,
            llvm::IntrusiveRefCntPtr<llvm::vfs::InMemoryFileSystem>& fs,
-           llvm::raw_pwrite_stream& stdout, llvm::raw_pwrite_stream& stderr)
+           FILE* input_stream, llvm::raw_pwrite_stream& output_stream,
+           llvm::raw_pwrite_stream& error_stream)
       -> ErrorOr<RunResult> override {
     CARBON_ASSIGN_OR_RETURN(auto prelude, installation_.ReadPreludeManifest());
     if (!is_no_prelude()) {
@@ -46,16 +47,16 @@ class ToolchainFileTest : public FileTestBase {
 
     Driver driver({.fs = fs,
                    .installation = &installation_,
-                   .input_stream = nullptr,
-                   .output_stream = &stdout,
-                   .error_stream = &stderr});
+                   .input_stream = input_stream,
+                   .output_stream = &output_stream,
+                   .error_stream = &error_stream});
     auto driver_result = driver.RunCommand(test_args);
     // If any diagnostics have been produced, add a trailing newline to make the
     // last diagnostic match intermediate diagnostics (that have a newline
     // separator between them). This reduces churn when adding new diagnostics
     // to test cases.
-    if (stderr.tell() > 0) {
-      stderr << '\n';
+    if (error_stream.tell() > 0) {
+      error_stream << '\n';
     }
 
     RunResult result{