[lld] [lld-macho] Implement ObjC category merging (-objc_category_merging) (PR #82928)

[via llvm-commits]

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+# REQUIRES: aarch64
+# RUN: rm -rf %t; split-file %s %t && cd %t
+
+## Create a dylib with a fake base class to link against
+# RUN: llvm-mc -filetype=obj -triple=arm64-ios-simulator -o a64_fakedylib.o a64_fakedylib.s
+# RUN: ld64.lld a64_fakedylib.o -o a64_fakedylib.dylib -dylib -arch arm64 -platform_version ios-simulator 14.0 15.0
+
+## Create our main testing dylib - linking against the fake dylib above
+# RUN: llvm-mc -filetype=obj -triple=arm64-ios-simulator -o merge_cat_minimal.o merge_cat_minimal.s
+# RUN: ld64.lld -dylib -o merge_cat_minimal_no_merge.dylib a64_fakedylib.dylib merge_cat_minimal.o -arch arm64 -platform_version ios-simulator 14.0 15.0
+# RUN: ld64.lld -dylib -o merge_cat_minimal_merge.dylib -objc_category_merging a64_fakedylib.dylib merge_cat_minimal.o -arch arm64 -platform_version ios-simulator 14.0 15.0
+
+## Now verify that the flag caused category merging to happen appropriatelly
+# RUN: llvm-objdump --objc-meta-data --macho merge_cat_minimal_no_merge.dylib | FileCheck %s --check-prefixes=NO_MERGE_CATS
+# RUN: llvm-objdump --objc-meta-data --macho merge_cat_minimal_merge.dylib | FileCheck %s --check-prefixes=MERGE_CATS
+
+#### Check merge categories enabled ###
+# Check that the original categories are not there
+MERGE_CATS-NOT: __OBJC_$_CATEGORY_MyBaseClass_$_Category01
+MERGE_CATS-NOT: __OBJC_$_CATEGORY_MyBaseClass_$_Category02
+
+# Check that the merged cateogry is there, in the correct format
+MERGE_CATS: __OBJC_$_CATEGORY_MyBaseClass_$_(Category01|Category02)
+MERGE_CATS: instanceMethods
+MERGE_CATS-NEXT: 24
+MERGE_CATS-NEXT: 2
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alx32 wrote:

TODO: also check merged methods. 

https://github.com/llvm/llvm-project/pull/82928