build: only generate compilation database for the nvim target (#20449)

Some tools like clang-tidy get confused and repeat unnecessary work if
there are multiple entries with the same name.

This will only generate a compilation database for cmake version 3.20
and above. Generating a compilation database is only necessary for
development, so we don't need to maintain compatibility with the minimum
required version.

Closes https://github.com/neovim/neovim/issues/10632

src/nvim/CMakeLists.txt

@@ -446,6 +446,11 @@ set(NVIM_EXEC_LINK_LIBRARIES ${NVIM_LINK_LIBRARIES} ${LUA_PREFERRED_LIBRARIES})
 add_executable(nvim ${NVIM_GENERATED_FOR_SOURCES} ${NVIM_GENERATED_FOR_HEADERS}
   ${NVIM_GENERATED_SOURCES} ${NVIM_SOURCES} ${NVIM_HEADERS}
   ${EXTERNAL_SOURCES} ${EXTERNAL_HEADERS})
+
+set_target_properties(nvim
+  PROPERTIES
+  EXPORT_COMPILE_COMMANDS ON)
+
 target_link_libraries(nvim ${NVIM_EXEC_LINK_LIBRARIES})
 install_helper(TARGETS nvim)
 if(MSVC)