Sema: Fix incorrect assert in TypeCheckProtocol.cpp.

We can't assume that we will always find a viable witness for a protocol
requirement when typechecking a `.swiftinterface` file. The assert that assumed
there would be a match could fail when building a broken interface or building
a valid interface with a broken SDK, causing crashes instead of emitting
diagnostics.

Resolves rdar://108688535

Author: tshortli

lib/Sema/TypeCheckProtocol.cpp

@@ -1504,11 +1504,12 @@ bool WitnessChecker::findBestWitness(
         if (SF->Kind == SourceFileKind::Interface) {
           auto match = matchWitness(ReqEnvironmentCache, Proto,
                                     conformance, DC, requirement, requirement);
-          assert(match.isViable());
-          numViable = 1;
-          bestIdx = matches.size();
-          matches.push_back(std::move(match));
-          return true;
+          if (match.isViable()) {
+            numViable = 1;
+            bestIdx = matches.size();
+            matches.push_back(std::move(match));
+            return true;
+          }
         }
       }
     }

validation-test/ParseableInterface/broken-optionset.swiftinterface

@@ -0,0 +1,36 @@
+// swift-interface-format-version: 1.0
+// swift-compiler-version: Swift version 5.0
+// swift-module-flags:
+
+// RUN: %empty-directory(%t)
+// RUN: not %target-swift-frontend -typecheck-module-from-interface -module-name Broken %s 2>%t/interface-verification-errors.txt
+// RUN: %FileCheck %s < %t/interface-verification-errors.txt
+
+import Swift
+
+// This type is meant to shadow the name of the module, creating an ambiguity.
+public struct Broken {}
+
+public struct BrokenOptions : Swift.OptionSet {
+	public let rawValue: Swift.Int
+	public static let none: Broken.BrokenOptions
+	public static let anOption: Broken.BrokenOptions
+	public init(rawValue: Swift.Int)
+	public typealias ArrayLiteralElement = Broken.BrokenOptions
+	public typealias Element = Broken.BrokenOptions
+	public typealias RawValue = Swift.Int
+}
+
+// CHECK: error: 'BrokenOptions' is not a member type of struct 'Broken.Broken'
+// CHECK: error: 'BrokenOptions' is not a member type of struct 'Broken.Broken'
+// CHECK: error: 'BrokenOptions' is not a member type of struct 'Broken.Broken'
+// CHECK: error: 'BrokenOptions' is not a member type of struct 'Broken.Broken'
+
+// CHECK: error: type 'BrokenOptions' does not conform to protocol 'OptionSet'
+// CHECK: error: type 'BrokenOptions' does not conform to protocol 'SetAlgebra'
+// CHECK: note: candidate would match if 'BrokenOptions.Element' was the same type as 'τ_0_0.Element'
+// CHECK: error: type 'BrokenOptions' does not conform to protocol 'ExpressibleByArrayLiteral'
+// CHECK: note: protocol requires nested type 'Element'; do you want to add it?
+// CHECK: note: protocol requires initializer 'init(_:)' with type 'S'
+// CHECK: note: protocol requires nested type 'ArrayLiteralElement'; do you want to add it?
+// CHECK: error: failed to verify module interface of 'Broken' due to the errors above; the textual interface may be broken by project issues, differences between compilers