all: add full LLVM 20 support

This switches the Espressif fork from LLVM 19 to LLVM 20, so we can use
the improvements made between those LLVM versions. It also better aligns
with the system-LLVM build method, which currently also defaults to LLVM
20.

builder/clang.cpp

@@ -60,7 +60,7 @@ bool tinygo_clang_driver(int argc, char **argv) {
 			}
 
 			// Create the actual diagnostics engine.
-			Clang->createDiagnostics();
+			Clang->createDiagnostics(*llvm::vfs::getRealFileSystem());
 			if (!Clang->hasDiagnostics()) {
 				return false;
 			}