[QoI] While merging equivalence classes don't forget to merge fixed types (if any)

Otherwise merging of the representative equivalence classes leaves constraint
system and graph disconnected, which might leave to incorrect solutions.

Author: xedin

lib/Sema/CSGen.cpp

@@ -71,6 +71,19 @@ static bool mergeRepresentativeEquivalenceClasses(ConstraintSystem &CS,
     auto rep2 = CS.getRepresentative(tyvar2);
 
     if (rep1 != rep2) {
+      auto fixedType2 = CS.getFixedType(rep2);
+
+      // If the there exists fixed type associated with the second
+      // type variable, and we simply merge two types together it would
+      // mean that portion of the constraint graph previously associated
+      // with that (second) variable is going to be disconnected from its
+      // new equivalence class, which is going to lead to incorrect solutions,
+      // so we need to make sure to re-bind fixed to the new representative.
+      if (fixedType2) {
+        CS.addConstraint(ConstraintKind::Bind, fixedType2, rep1,
+                         rep1->getImpl().getLocator());
+      }
+
       CS.mergeEquivalenceClasses(rep1, rep2, /*updateWorkList*/ false);
       return true;
     }

validation-test/compiler_crashers/28588-unreachable-executed-at-swift-lib-sema-csapply-cpp-5770.swift renamed to validation-test/compiler_crashers_fixed/28588-unreachable-executed-at-swift-lib-sema-csapply-cpp-5770.swift

@@ -5,5 +5,5 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+// RUN: not %target-swift-frontend %s -emit-ir
 {$0=($0={