Meta: Automatically generate a compilation database for clangd

This is similar to how we set CMAKE_EXPORT_COMPILE_COMMANDS in the CMake
build. This adds about 5-10ms to a clean `gn gen` on my machine.

.gn

@@ -1,4 +1,5 @@
 buildconfig = "//Meta/gn/build/BUILDCONFIG.gn"
 exec_script_whitelist = []
+export_compile_commands = [ ":*" ]
 script_executable = "python3"
 secondary_source = "//Meta/gn/secondary/"