Fix precedence of "await" and some erroneous test cases

Fix precedence of "await" and some erroneous test cases

Summary:
While working on getting the full-fidelity parser behaviour for -> correct, I noticed that one of the test cases was wrong in two ways. The test was intended to determine whether the "array" built-in was being parsed correctly, but the case was wrong; it was:

    await $x->array( ... )

The intention of the test was to have the right side of the -> parsed as an expression, but a name immediately following -> should always be parsed as the member of the receiver. That is, this should be

   await ( ($x->array) ( ... ) )

and not

    await ( $x-> ( array ( ... ) ) )

I changed the -> to a . so that we do not have this problem.

Second, the "await" operator should be the lowest precedence operator, but for some reason which escapes me, I had it on the precedence list with casts, which is wrong. This meant that another test case was getting the wrong answer:

    await $x + $y

should be parsed as

    await ($x + $y)

and not

    (await $x) + $y

I changed the precedence and fixed the test case.

At some point we should get the precedence rules in a table in the spec, but that can come later.

Reviewed By: jamesjwu

Differential Revision: D3753588

fbshipit-source-id: 57d140fd1da4461f63fab5a84b668a626e9e4a1a