fix(language-service): correctly parse expressions in an attribute

[ayazhafiz]

Currently, the language service provides completions in a template node
attribute by first checking if the attribute contains template bindings
to provide completions for, and then providing completions for the
expression in the attribute.

In the latter case, the expression AST was being constructed
"synthetically" inside the language service, in particular declaring the
expression to be a PropertyRead with an implicit receiver.
Unfortunately, this AST can be incorrect if the expression is actually a
property read on a component property receiver (e.g. when reading
key in the expression obj.key, obj is the receiver).

The fix is pretty simple - rather than a synthetic construction of the
AST, ask the expression parser to parse the expression in the attribute.

Fixes angular/vscode-ng-language-service#523

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix

Does this PR introduce a breaking change?

• Yes
• No

[kyliau]

thank you, this is really clean.

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