This adds three new assertion macros:
* `ASSERT` - always compiled in, always checked
* `CONDITIONAL_ASSERT` - always compiled in, checked whenever the `-compiler-assertions` flag is provided
* `DEBUG_ASSERT` - only compiled into debug builds, always checked when compiled in (functionally the same as Standard C `assert`)
The new `-compiler-assertions` flag is recognized by both `swift-frontend` and
`swiftc`.
The goal is to eventually replace every use of `assert` in the compiler with one of the above:
* Most assertions will use `ASSERT` (most assertions should always be present and checked, even in release builds)
* Expensive assertions can use `CONDITIONAL_ASSERT` to be suppressed by default
* A few very expensive and/or brittle assertions can use `DEBUG_ASSERT` to be compiled out of release builds
This should:
* Improve quality by catching errors earlier,
* Accelerate compiler triage and debugging by providing more accurate crash dumps by default, and
* Allow compiler engineers and end users alike to add `-compiler-assertions` to get more accurate failure diagnostics with any compiler
