Add a per-run cache for member symbols

We are all used to calls to `definitions.PredefModule`, or
`defintions.Predef_???` to grab the symbol of some well known
entity. But you'll notice that some of these are lazy vals,
and others are defs.

Why is this so? In the presentation compiler, a member like
`Predef.???` will be assigned a new symbol after the user
browses into `Predef.scala`.

Mistakenly using vals in definitions leads to subtle
IDE bugs like SI-7678. We are able to trigger these
situations in tests, as noted in the comments of that issue.

Changing the vals to defs, on the other hand, has a performance
penalty. Some schemes to workaround this have shown up:
cache them per-implicit search, or compare method names and owners
rather than symbols on hot paths in the type checker.

This commit introduces a facility to cache these sort of symbols
per-run, and uses it to check for `Predef.conforms` and
and for the class/type tag materializers.

A followup pull request (WIP: https://github.com/retronym/scala/compare/ticket/7678-2)
will expand the use of to address the widespread and unsafe caching
of member symbols that I found while investigating SI-7678.

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