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

[sbc100]

Summary

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.

[sbc100]

cleanup

lld/ELF/InputSection.cpp

 }
 
 // Returns the offset in an output section for a given input offset.
 // Because contents of a mergeable section is not contiguous in output,
 // it is not just an addition to a base output 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));
-  uint64_t addend = offset - piece.inputOff;
-  return piece.outputOff + addend;
+  const SectionPiece *piece = getSectionPiece(offset);
+  uint64_t addend = offset - piece->inputOff;
+  return piece->outputOff + addend;
 }
 
 template InputSection::InputSection(ObjFile<ELF32LE> &, const ELF32LE::Shdr &,
                                     StringRef);
 template InputSection::InputSection(ObjFile<ELF32BE> &, const ELF32BE::Shdr &,
                                     StringRef);
 template InputSection::InputSection(ObjFile<ELF64LE> &, const ELF64LE::Shdr &,
                                     StringRef);

lld/test/ELF/merge-string.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 // RUN: ld.lld -O 2 %t.o -o %t.so -shared
 // RUN: llvm-readobj -S --section-data --symbols %t.so | FileCheck %s
 // RUN: ld.lld -O 1 %t.o -o %t.so -shared
 // RUN: llvm-readobj -S --section-data --symbols %t.so | FileCheck --check-prefix=NOTAIL %s
 // 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
 
 // CHECK:      Name:    .rodata1
 // CHECK-NEXT: Type:    SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC