[pfff] unbreak parsing `$$variable` with a quick lexer hack
[pfff] unbreak parsing `$$variable` with a quick lexer hack

Summary:
Managed to break this in D3005435. The fix is to explicitly
look for `$$name` cases in the lexer, and push the dollar
token separate from the `VARIABLE`. This way, the parser
gets the tokens it would see prior to D3005435.

Test Plan:
  ~/pfff $ make test
  ~/pfff $ ./pfff -dump_php ./tests/php/parsing/dynamic_var.php

**Output**

  [StmtList(
   [ExprStmt(
         Assign(IdVar(DName(("o", i_1)), Ref(NoScope)), i_2,
                 Sc(C(String(("foo", i_3))))), i_4);
                     ExprStmt(
                           Assign(IdVar(DName(("foo", i_5)), Ref(NoScope)), i_6,
                                   Sc(C(Int(("42", i_7))))), i_8);
                                       Echo(i_9, [Left(Deref(i_10, IdVar(DName(("o", i_11)), Ref(NoScope))))],
                                             i_12)]); FinalDef(i_13)]

(Previous output after D3005435 was to barf :()

Reviewers: logozzo, dreeves

Differential Revision: https://phabricator.fb.com/D3089181

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