[Build] Consider Swift on Clang Modules as build order dependency

Currently swiftpm fatalError() when some swift package is dependency
on a package containing c languages or mixed languages. This shouldn't
error out. Swift will not be importable to c modules but there could be
swift modules in that package which want to use some external deps.
Considering it as a build order dependency should be ok.

Author: aciidgh

Fixtures/DependencyResolution/External/CUsingCDep2/Foo/Sources/Foo.c renamed to Fixtures/DependencyResolution/External/CUsingCDep2/Foo/Sources/Foo/Foo.c

@@ -64,6 +64,8 @@ struct ClangModuleBuildMetadata {
                 return product.targetName
             case let module as CModule:
                 return module.targetName
+            case let module as SwiftModule:
+                return module.targetName
             default:
                 fatalError("ClangModule \(self.module) can't have \(module) as a dependency.")
             }

Fixtures/DependencyResolution/External/CUsingCDep2/Foo/Sources/include/Foo/Foo.h renamed to Fixtures/DependencyResolution/External/CUsingCDep2/Foo/Sources/Foo/include/Foo/Foo.h

@@ -11,7 +11,7 @@
 import XCTest
 
 import Basic
-import Build
+@testable import Build
 import PackageDescription
 import PackageGraph
 import PackageModel
@@ -55,8 +55,17 @@ final class DescribeTests: XCTestCase {
         }
     }
 
+    func testClangModuleCanHaveSwiftDep() throws {
+        let clangModule = try ClangModule(name: "ClangModule", sources: Sources(paths: [], root: .root))
+        let swiftModule = try SwiftModule(name: "SwiftModule", sources: Sources(paths: [], root: .root))
+        clangModule.dependencies = [swiftModule]
+        let buildMeta = ClangModuleBuildMetadata(module: clangModule, prefix: .root, otherArgs: [])
+        XCTAssertEqual(buildMeta.inputs, ["<SwiftModule.module>"])
+    }
+
     static var allTests = [
         ("testDescribingNoModulesThrows", testDescribingNoModulesThrows),
         ("testDescribingCModuleThrows", testDescribingCModuleThrows),
+        ("testClangModuleCanHaveSwiftDep", testClangModuleCanHaveSwiftDep),
     ]
 }