Reduce overhead of `.expectationChecked` event handling in `#expect()`.

[grynspan]

This PR refactors the implementation of #expect() and #require() a bit such that they don't incur more than minimal overhead posting .expectationChecked events if nobody is listening for them (which, currently, nobody is.)

We considered removing .expectationChecked outright, but XCTest has historically had a number of requests for a way to observe calls to XCTAssert() etc. even when they pass, so we opted not to remove the event kind at this time.

This PR also introduces a cache for fully-qualified type names so that we don't need to call into the runtime to get them as often.

Overall speedup is approximately 90% or 11x. Test time for a tight loop of 1,000,000 #expect() calls goes from 5.898893 seconds down to 0.515558291 seconds (as measured on my work computer.)

Resolves rdar://133517028.

Checklist:

• [x] Code and documentation should follow the style of the Style Guide.
• [x] If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci please test

[grynspan]

@swift-ci please test

[grynspan]

@swift-ci please test