Fix the checking of synthesizable conformances under a where clause

[kyouko-taiga]

Now we can write this:

type Box<T> {
  public let contents: T
  public memberwise init
}

conformance Box: Equatable where T: Equatable {}

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.75%. Comparing base (e47c274) to head (fb2f60c).
Report is 5 commits behind head on main.

Files	Patch %	Lines
Sources/FrontEnd/TypeChecking/TypeChecker.swift	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1567      +/-   ##
==========================================
+ Coverage   87.67%   87.75%   +0.08%     
==========================================
  Files         373      373              
  Lines       22620    22621       +1     
==========================================
+ Hits        19831    19852      +21     
+ Misses       2789     2769      -20     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.