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

[Jan Svoboda via cfe-commits]

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@@ -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;
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jansvoboda11 wrote:

Instead of setting `NumThreads` for the P1689 format, can we check if size of `Inputs` below and avoid creating the thread pool for any single-compilation database?

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