Better detect if we're in a pattern before suggesting completions for `let` and `var`

[hamishknight]

As noted in #67704 we currently suggest let and var in expression position due to the fact that we initially parse patterns as expressions. We ought to be able to detect if we're in a pattern context though, and only suggest let and var in that case.

rdar://113330201

[XaurDesu]

Is there any code that would give us an indication that we're in a pattern context already, or should that be written for this issue?