[lld-macho] Fix -no_objc_category_merging flag (llvm#98238)

`-no_objc_category_merging` flag was behaving like
`-objc_category_merging` - i.e. acting to enable category merging.
This is because we were using `hasArg` instead of `hasFlag` to test for
it. Fix this and add test to ensure it behaves as expected.

Author: alx32

lld/MachO/Driver.cpp

@@ -1989,7 +1989,8 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
 
     // Category merging uses "->live = false" to erase old category data, so
     // it has to run after dead-stripping (markLive).
-    if (args.hasArg(OPT_objc_category_merging, OPT_no_objc_category_merging))
+    if (args.hasFlag(OPT_objc_category_merging, OPT_no_objc_category_merging,
+                     false))
       objc::mergeCategories();
 
     // ICF assumes that all literals have been folded already, so we must run

lld/test/MachO/objc-category-merging-complete-test.s

@@ -8,9 +8,13 @@
 
 # RUN: llvm-mc -filetype=obj -triple=arm64-apple-macos -o a64_file2.o a64_file2.s
 # RUN: %lld -arch arm64 -o a64_file2_no_merge.exe a64_file1.dylib a64_file2.o
+# RUN: %lld -arch arm64 -o a64_file2_no_merge_v2.exe a64_file1.dylib a64_file2.o -no_objc_category_merging
+# RUN: %lld -arch arm64 -o a64_file2_no_merge_v3.exe a64_file1.dylib a64_file2.o -objc_category_merging -no_objc_category_merging
 # RUN: %lld -arch arm64 -o a64_file2_merge.exe -objc_category_merging a64_file1.dylib a64_file2.o
 
 # RUN: llvm-objdump --objc-meta-data --macho a64_file2_no_merge.exe | FileCheck %s --check-prefixes=NO_MERGE_CATS
+# RUN: llvm-objdump --objc-meta-data --macho a64_file2_no_merge_v2.exe | FileCheck %s --check-prefixes=NO_MERGE_CATS
+# RUN: llvm-objdump --objc-meta-data --macho a64_file2_no_merge_v3.exe | FileCheck %s --check-prefixes=NO_MERGE_CATS
 # RUN: llvm-objdump --objc-meta-data --macho a64_file2_merge.exe | FileCheck %s --check-prefixes=MERGE_CATS
 
 ############ Test merging multiple categories into the base class ############