[lld][ELF] Removing redundant cast. NFC.

Also a couple of minor cleanups in merge-string.s:
- fix inconsistent use of tabs
- use `.p2align` rather than `.align` since `.p2align` works the
  same on all platforms (the meaning of align seems to differ
  between platforms according to `AlignmentIsInBytes`.

I noticed these potential cleanups while porting SHF_STRINGS support to
wasm-ld.

Differential Revision: https://reviews.llvm.org/D97647

Author: sbc100

lld/ELF/InputSection.cpp

@@ -1429,8 +1429,7 @@ SectionPiece *MergeInputSection::getSectionPiece(uint64_t offset) {
 uint64_t MergeInputSection::getParentOffset(uint64_t offset) const {
   // If Offset is not at beginning of a section piece, it is not in the map.
   // In that case we need to search from the original section piece vector.
-  const SectionPiece &piece =
-      *(const_cast<MergeInputSection *>(this)->getSectionPiece (offset));
+  const SectionPiece &piece = *getSectionPiece(offset);
   uint64_t addend = offset - piece.inputOff;
   return piece.outputOff + addend;
 }

lld/test/ELF/merge-string.s

@@ -7,16 +7,16 @@
 // RUN: ld.lld -O 0 %t.o -o %t.so -shared
 // RUN: llvm-readobj -S --section-data --symbols %t.so | FileCheck --check-prefix=NOMERGE %s
 
-        .section	.rodata1,"aMS",@progbits,1
-	.asciz	"abc"
+        .section        .rodata1,"aMS",@progbits,1
+        .asciz  "abc"
 foo:
-	.ascii	"a"
+        .ascii  "a"
 bar:
         .asciz  "bc"
         .asciz  "bc"
 
         .section        .rodata2,"aMS",@progbits,2
-        .align  2
+        .p2align  1
 zed:
         .short  20
         .short  0