[clang] [NFC] [C++20] [Modules] [P1689] [Scanner] Don't use thread pool in P1689 per file mode (PR #84285)

[Chuanqi Xu via cfe-commits]

================
@@ -744,6 +744,9 @@ getCompilationDataBase(int argc, char **argv, std::string &ErrorMessage) {
     return nullptr;
   }
 
+  // Only 1 threads is required if P1689 per file mode.
+  NumThreads = 1;
----------------
ChuanqiXu9 wrote:

Oh, sorry. I don't know why I missed this somehow. This should be done now.

https://github.com/llvm/llvm-project/pull/84285