[Clang] Mark RetainCommentsFromSystemHeaders as compatible language options

Motivated by comments in clangd/clangd#1293

And RetainCommentsFromSystemHeaders shouldn't affect the compatibleness
anyway.

Author: ChuanqiXu9

clang/include/clang/Basic/LangOptions.def

@@ -430,7 +430,7 @@ LANGOPT(ApplePragmaPack, 1, 0, "Apple gcc-compatible #pragma pack handling")
 
 LANGOPT(XLPragmaPack, 1, 0, "IBM XL #pragma pack handling")
 
-LANGOPT(RetainCommentsFromSystemHeaders, 1, 0, "retain documentation comments from system headers in the AST")
+COMPATIBLE_LANGOPT(RetainCommentsFromSystemHeaders, 1, 0, "retain documentation comments from system headers in the AST")
 
 LANGOPT(APINotes, 1, 0, "use external API notes")
 LANGOPT(APINotesModules, 1, 0, "use module-based external API notes")

clang/test/Modules/diff-retain-comments-from-system-headers-flag.cppm

@@ -0,0 +1,13 @@
+// RUN: rm -rf %t
+// RUN: mkdir %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/a.cppm -emit-module-interface -o %t/a.pcm -fretain-comments-from-system-headers
+// RUN: %clang_cc1 -std=c++20 %t/b.cpp -fmodule-file=a=%t/a.pcm -verify -fsyntax-only
+
+//--- a.cppm
+export module a;
+
+//--- b.cpp
+// expected-no-diagnostics
+import a;