Stop passing -no_objc_category_merging to the linker on Darwin.

ObjC category merging is a really useful optimization, so we should stop doing that. This flag was added in the legacy driver as a workaround in 2014:

commit 9a1cf1af1691edc5da237b88be0b0fac548a5a09
Author: Greg Parker <[email protected]>
Date:   Wed Feb 19 09:22:38 2014 +0000

And whatever issue it was working around should now be resolved.

Resolves rdar://84099601

Author: artemcm

Sources/SwiftDriver/Jobs/DarwinToolchain+LinkerSupport.swift

@@ -418,8 +418,6 @@ extension DarwinToolchain {
       sdkPath: targetInfo.sdkPath?.path
     )
 
-    commandLine.appendFlag("-no_objc_category_merging")
-
     // These custom arguments should be right before the object file at the
     // end.
     try commandLine.appendAllExcept(

Tests/SwiftDriverTests/JobExecutorTests.swift

@@ -183,7 +183,7 @@ final class JobExecutorTests: XCTestCase {
           "-lobjc", "-lSystem", "-arch", .flag(hostTriple.archName),
           "-L", .path(.absolute(try toolchain.resourcesDirectory.get())),
           "-L", .path(.absolute(try toolchain.sdkStdlib(sdk: toolchain.sdk.get()))),
-          "-rpath", "/usr/lib/swift", "-macosx_version_min", "10.14.0", "-no_objc_category_merging", "-o",
+          "-rpath", "/usr/lib/swift", "-macosx_version_min", "10.14.0", "-o",
           .path(.relative(RelativePath("main"))),
         ],
         inputs: [
@@ -254,7 +254,7 @@ final class JobExecutorTests: XCTestCase {
           "-lobjc", "-lSystem", "-arch", .flag(hostTriple.archName),
           "-L", .path(.absolute(try toolchain.resourcesDirectory.get())),
           "-L", .path(.absolute(try toolchain.sdkStdlib(sdk: toolchain.sdk.get()))),
-          "-rpath", "/usr/lib/swift", "-macosx_version_min", "10.14.0", "-no_objc_category_merging",
+          "-rpath", "/usr/lib/swift", "-macosx_version_min", "10.14.0",
           "-o", .path(.absolute(exec)),
         ],
         inputs: [