cmake: prefer system llvm library

externals/CMakeLists.txt

@@ -158,6 +158,9 @@ if (YUZU_USE_EXTERNAL_VULKAN_HEADERS)
     add_subdirectory(Vulkan-Headers EXCLUDE_FROM_ALL)
 endif()
 
-add_library(demangle STATIC)
-target_include_directories(demangle PUBLIC ./demangle)
-target_sources(demangle PRIVATE demangle/ItaniumDemangle.cpp)
+if (NOT TARGET LLVM::Demangle)
+    add_library(demangle STATIC)
+    target_include_directories(demangle PUBLIC ./demangle)
+    target_sources(demangle PRIVATE demangle/ItaniumDemangle.cpp)
+    add_library(LLVM::Demangle ALIAS demangle)
+endif()

externals/demangle/ItaniumDemangle.cpp

@@ -11,8 +11,8 @@
 // file does not yet support:
 //   - C++ modules TS
 
-#include "Demangle.h"
-#include "ItaniumDemangle.h"
+#include "llvm/Demangle/Demangle.h"
+#include "llvm/Demangle/ItaniumDemangle.h"
 
 #include <cassert>
 #include <cctype>