[clang-tools-extra] [clangd] [C++20] [Modules] Introduce initial support for C++20 Modules (PR #66462)

[kadir çetinkaya via cfe-commits]

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@@ -51,6 +52,9 @@
 
 namespace clang {
 namespace clangd {
+
+extern llvm::cl::opt<bool> ExperimentalModulesSupport;
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kadircet wrote:

we don't need this declaration for the command line flag, we're already passing this through `ClangdLSPServer::Options`.

https://github.com/llvm/llvm-project/pull/66462