Add sentinel Eof token at the end of the tokenized buffer. (#388)

This has two goals:

1) It allows us to simplify and remove special cases from the parser:
   when we expect a particular token next, we can just check for it
   without needing a special case for end-of-file.

2) It gives us a token to use as a position when emitting diagnostics at
   the end of the file.

Centralize all updating of `position` to `Consume` and `SkipTo`, so that we can in a single place ensure that we never go past the EOF token.

parser/parse_node_kind.def

@@ -21,5 +21,6 @@ CARBON_PARSE_NODE_KIND(FunctionDeclaration)
 CARBON_PARSE_NODE_KIND(Identifier)
 CARBON_PARSE_NODE_KIND(ParameterListEnd)
 CARBON_PARSE_NODE_KIND(ParameterList)
+CARBON_PARSE_NODE_KIND(FileEnd)
 
 #undef CARBON_PARSE_NODE_KIND