Fix GCC Wimplicit-fallthrough warnings. NFC.

Author: RKSimon

clang/lib/Sema/Sema.cpp

@@ -2865,6 +2865,7 @@ bool FunctionEffectDiff::shouldDiagnoseConversion(
       // matching is better.
       return true;
     }
+    llvm_unreachable("Unhandled FunctionEffectDiff::Kind enum");
   case FunctionEffect::Kind::Blocking:
   case FunctionEffect::Kind::Allocating:
     return false;
@@ -2890,6 +2891,7 @@ bool FunctionEffectDiff::shouldDiagnoseRedeclaration(
       // All these forms of mismatches are diagnosed.
       return true;
     }
+    llvm_unreachable("Unhandled FunctionEffectDiff::Kind enum");
   case FunctionEffect::Kind::Blocking:
   case FunctionEffect::Kind::Allocating:
     return false;
@@ -2921,6 +2923,7 @@ FunctionEffectDiff::shouldDiagnoseMethodOverride(
     case Kind::ConditionMismatch:
       return OverrideResult::Warn;
     }
+    llvm_unreachable("Unhandled FunctionEffectDiff::Kind enum");
 
   case FunctionEffect::Kind::Blocking:
   case FunctionEffect::Kind::Allocating: