Generic methods declared in protocols (and extensions thereof) cannot
be used on existential values, because there is no way to specialize
them for all potential types. Diagnose such cases in Embedded Swift
mode and via `-Wwarning EmbeddedRestrictions`.
This adds a bunch more warnings to the standard library that we'll
need to clean up, probably by `#if`'ing more code out.
Part of rdar://119383905.
1. move embedded diagnostics out of the PerformanceDiagnostics pass. It was completely separated from the other logic in this pass, anyway.
2. rewrite it in swift
3. fix several bugs, that means: missed diagnostics, which led to IRGen crashes
* look at all methods in witness tables, including base protocols and associated conformances
* visit all functions in the call tree, including generic functions with class bound generic arguments
* handle all instructions, e.g. concurrency builtins
4. improve error messages by adding meaningful call-site information. For example:
* if the error is in a specialized function, report where the generic function is originally specialized with concrete types
* if the error is in a protocol witness method, report where the existential is created
