[ELF] -r: keep sh_entsize for SHF_MERGE sections with relocations

Follow-up to the NFC refactoring
43e3871 and test cleanup
3cecf17.

SHF_MERGE sections with relocations are handled as InputSection (without
duplicate elimination). The output section retains the original
sh_entsize in non-relocatable links. This patch ports the behavior for
relocatable links as well.

ClangBuiltLinux/linux#2057

Author: MaskRay

lld/ELF/InputFiles.cpp

@@ -904,7 +904,7 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats,
       // is acceptable because section merging is optional.
       if (auto *ms = dyn_cast<MergeInputSection>(s)) {
         s = makeThreadLocal<InputSection>(ms->file, ms->name, ms->type,
-                                          ms->flags, ms->addralign, 0,
+                                          ms->flags, ms->addralign, ms->entsize,
                                           ms->contentMaybeDecompress());
         sections[info] = s;
       }

lld/test/ELF/merge-reloc.s

@@ -12,9 +12,9 @@
 # CHECK-NEXT:  [ 1] .text             PROGBITS        0000000000000000 000040 000000 00  AX  0   0  4
 # CHECK-NEXT:  [ 2] .rodata.1         PROGBITS        0000000000000000 000040 000004 04  AM  0   0  4
 # CHECK-NEXT:  [ 3] .rodata.2         PROGBITS        0000000000000000 000048 000008 08  AM  0   0  8
-# CHECK-NEXT:  [ 4] .rodata.cst8      PROGBITS        0000000000000000 000050 000010 00  AM  0   0  1
+# CHECK-NEXT:  [ 4] .rodata.cst8      PROGBITS        0000000000000000 000050 000010 08  AM  0   0  1
 # CHECK-NEXT:  [ 5] .rela.rodata.cst8 RELA            0000000000000000 000068 000030 18   I  9   4  8
-# CHECK-NEXT:  [ 6] .cst4             PROGBITS        0000000000000000 000060 000008 00  AM  0   0  1
+# CHECK-NEXT:  [ 6] .cst4             PROGBITS        0000000000000000 000060 000008 04  AM  0   0  1
 # CHECK-NEXT:  [ 7] .rela.cst4        RELA            0000000000000000 000098 000030 18   I  9   6  8
 
 # OBJDUMP:      Contents of section .rodata.1:
@@ -26,7 +26,7 @@
 # OBJDUMP:      Contents of section .cst4:
 # OBJDUMP-NEXT:  0000 00000000 00000000                    ........
 
-# CHECK-PDE: [ 2] .cst4             PROGBITS        0000000000200140 000140 000008 00  AM  0   0  1
+# CHECK-PDE: [ 2] .cst4             PROGBITS        0000000000200140 000140 000008 04  AM  0   0  1
 
 foo: