LiteralSupport: Don't assert() on invalid input

When using clangd, it's possible to trigger assertions in
NumericLiteralParser and CharLiteralParser when switching git branches.
This commit removes the initial asserts on invalid input and replaces
those asserts with the error handling mechanism from those respective
classes instead. This allows clangd to gracefully recover without
crashing.

See https://github.com/clangd/clangd/issues/888 for more information
on the clangd crashes.