[clang] [Frontend] Move -mllvm and loading of plugins to address a fixme. NFC (PR #192476)

[Aaron Ballman via cfe-commits]

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@@ -418,6 +418,8 @@ void dependencies::initializeScanCompilerInstance(
     DiagnosticConsumer *DiagConsumer, DependencyScanningService &Service,
     IntrusiveRefCntPtr<DependencyScanningWorkerFilesystem> DepFS) {
   ScanInstance.setBuildingModule(false);
+  // LLVM options are not going to affect dependency scanning.
+  ScanInstance.getFrontendOpts().LLVMArgs.clear();
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AaronBallman wrote:

Is this always true? e.g., do we need the dependency scanner to look for things which may be part of the source but not passed to the compiler frontend, like Windows resource files (.rc)? 

https://github.com/llvm/llvm-project/pull/192476