Fix lexer rules for XHP identifiers

Summary: This fixes how the typechecker's lexer deals with XHP identifiers to match
HHVM. Most notably, XHP identifiers are not allowed to end with a hyphen or
a colon.

Reviewed By: @int3

Differential Revision: D1739223

Signature: t1:1739223:1418616156:87575bcda7d550bec63584c14b373549fe5c3831