LibCpp: Add the beginning of a C++ parser

This parser will be used by the C++ langauge server to provide better
auto-complete (& maybe also other things in the future).

It is designed to be error tolerant, and keeps track of the position
spans of the AST nodes, which should be useful later for incremental
parsing.

Meta/CMake/all_the_debug_macros.cmake

@@ -159,6 +159,7 @@ set(UPDATE_COALESCING_DEBUG ON)
 set(VOLATILE_PAGE_RANGES_DEBUG ON)
 set(WSMESSAGELOOP_DEBUG ON)
 set(GPT_DEBUG ON)
+set(CPP_DEBUG ON)
 
 # False positive: DEBUG is a flag but it works differently.
 # set(DEBUG ON)