SI-7548 Test to demonstrate residual exploratory typing bug

We shouldn't observe tree types under silent mode.

The enclosed test is a standalone version of `1 + BigInt(2)`,
a standard example of exploratory typing in Scala. Once we determine
that none of the `+` methods in `Int` accepts (possibly implicitly
coerced `BigInt`), we have to backtrack and look for a view from
`Int => { +(_: BigInt): ? }`.

The next commit will correct the problem.
SI-7458 Pres. compiler must not observe trees in silent mode

Otherwise we can think that `+` in `1 + BigInt(2)` refers
to a method in `Int`.

In general, this protects the IDE from observing results from
"exploratory" typing which is discarded as the compiler backtracks
to another possibility.

This protection subsumes the condition that checked for overloaded
types: presentation/t7458 now passes without this.
Merge pull request #1 from retronym/review/3177

Check for silent mode, rather than for overloaded types