[lld-macho][nfc] Rename MergedOutputSection to ConcatOutputSection

The ELF format has the concept of merge sections (marked by SHF_MERGE),
which contain data that can be safely deduplicated. The Mach-O
equivalents are called literal sections (marked by S_CSTRING_LITERALS or
S_{4,8,16}BYTE_LITERALS). While the Mach-O format doesn't use the word
'merge', to avoid confusion, I've renamed our MergedOutputSection to
ConcatOutputSection. I believe it's a more descriptive name too.

This renaming sets the stage for {D102964}.

Reviewed By: #lld-macho, alexshap

Differential Revision: https://reviews.llvm.org/D102971