[test] Test that the prebuilt cache is ignored if no architecture matches

Just add a missing test to make sure that, if there's a prebuilt module
for a different architecture inside a `.swiftmodule` director, it's
ignored and the interface is used instead.

rdar://48422044

Author: Harlan Haskins

test/ParseableInterface/ModuleCache/prebuilt-module-cache-unusable.swift

@@ -0,0 +1,32 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/MCP)
+// RUN: %empty-directory(%t/prebuilt-cache/Lib.swiftmodule)
+// RUN: %empty-directory(%t/include/Lib.swiftmodule)
+// RUN: cp %S/Inputs/prebuilt-module-cache/Lib.swiftinterface %t/include/Lib.swiftmodule/%target-cpu.swiftinterface
+
+// Prebuild a module for the current target CPU, and put it in the prebuilt cache under some imaginary CPU.
+// RUN: sed -e 's/FromInterface/FromPrebuilt/g' %t/include/Lib.swiftmodule/%target-cpu.swiftinterface | %target-swift-frontend -parse-stdlib -module-cache-path %t/MCP -emit-module-path %t/prebuilt-cache/Lib.swiftmodule/leg128.swiftmodule - -module-name Lib
+
+// Make sure that, if there's a module for a different architecture
+// present in the prebuilt cache, it's ignored and the module is
+// rebuilt from an interface.
+
+// RUN: not %target-swift-frontend -typecheck -module-cache-path %t/MCP -sdk %t/include -I %t/include -prebuilt-module-cache-path %t/prebuilt-cache %s 2>&1 | %FileCheck %s --check-prefix FROM-INTERFACE
+// RUN: %empty-directory(%t/MCP)
+
+// Make sure it works fine if the module is for this architecture.
+// RUN: mv %t/prebuilt-cache/Lib.swiftmodule/leg128.swiftmodule %t/prebuilt-cache/Lib.swiftmodule/%target-swiftmodule-name
+// RUN: not %target-swift-frontend -typecheck -module-cache-path %t/MCP -sdk %t/include -I %t/include -prebuilt-module-cache-path %t/prebuilt-cache %s 2>&1 | %FileCheck %s --check-prefix FROM-PREBUILT
+
+// Now make sure it works if there's nothing in the prebuilt cache
+// RUN: %empty-directory(%t/prebuilt-cache/Lib.swiftmodule)
+// RUN: %empty-directory(%t/MCP)
+
+// RUN: not %target-swift-frontend -typecheck -module-cache-path %t/MCP -sdk %t/include -I %t/include -prebuilt-module-cache-path %t/prebuilt-cache %s 2>&1 | %FileCheck %s --check-prefix FROM-INTERFACE
+
+import Lib
+
+struct X {}
+let _: X = Lib.testValue
+// FROM-PREBUILT: [[@LINE-1]]:16: error: cannot convert value of type 'FromPrebuilt' to specified type 'X'
+// FROM-INTERFACE: [[@LINE-2]]:16: error: cannot convert value of type 'FromInterface' to specified type 'X'