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

[Vassil Vassilev via cfe-commits]

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@@ -945,19 +946,36 @@ bool CompilerInstance::InitializeSourceManager(const FrontendInputFile &Input,
 // High-Level Operations
 
 void CompilerInstance::PrepareForExecution() {
+  if (PreparedForExecution)
+    return;
+  PreparedForExecution = true;
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vgvassilev wrote:

Because we need to make sure `PreparedForExecution` is called just once. It can be called twice for some tools which rely on clang to bootstrap and then call `ExecuteAction`, iirc.

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