Support resources in ObjC++ code (#3828)

Resources weren't supported in ObjC++ code so far, but there wasn't a good reason for that.

rdar://83058216

Author: neonichu

Fixtures/Resources/Simple/Package.swift

@@ -17,5 +17,12 @@ let package = Package(
                 .copy("foo.txt"),
             ]
         ),
+
+        .target(
+            name: "CPPResource",
+            resources: [
+                .copy("foo.txt"),
+            ]
+        ),
     ]
 )

Fixtures/Resources/Simple/Sources/CPPResource/foo.txt

@@ -0,0 +1 @@
+foo

Fixtures/Resources/Simple/Sources/CPPResource/main.mm

@@ -0,0 +1,12 @@
+#import <Foundation/Foundation.h>
+
+int main(int argc, const char * argv[]) {
+    @autoreleasepool {
+        NSBundle *bundle = SWIFTPM_MODULE_BUNDLE;
+        NSString *foo = [bundle pathForResource:@"foo" ofType:@"txt"];
+        NSData *data = [NSFileManager.defaultManager contentsAtPath:foo];
+        NSString *contents = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
+        printf("%s", contents.UTF8String);
+    }
+    return 0;
+}

Sources/Build/BuildPlan.swift

@@ -471,9 +471,17 @@ public final class ClangTargetBuildDescription {
         headerFileStream <<< """
         #import <Foundation/Foundation.h>
 
+        #if __cplusplus
+        extern "C" {
+        #endif
+
         NSBundle* \(target.c99name)_SWIFTPM_MODULE_BUNDLE(void);
 
         #define SWIFTPM_MODULE_BUNDLE \(target.c99name)_SWIFTPM_MODULE_BUNDLE()
+
+        #if __cplusplus
+        }
+        #endif
         """
         let headerFile = derivedSources.root.appending(component: "resource_bundle_accessor.h")
         self.resourceAccessorHeaderFile = headerFile

Sources/PackageModel/Sources.swift

@@ -46,7 +46,7 @@ public struct Sources: Codable {
             guard let ext = $0.extension else {
                 return false
             }
-            return ext == SupportedLanguageExtension.m.rawValue
+            return ext == SupportedLanguageExtension.m.rawValue || ext == SupportedLanguageExtension.mm.rawValue
         })
     }
 }

Tests/FunctionalTests/ResourcesTests.swift

@@ -21,6 +21,7 @@ class ResourcesTests: XCTestCase {
             // Objective-C module requires macOS
             #if os(macOS)
             executables.append("SeaResource")
+            executables.append("CPPResource")
             #endif
 
             for execName in executables {