[llvm-size] Fix missing file name for darwin output format with non-Mach-O

llvm-size falls back to printing in Berkeley format, if --format=darwin is specified and a non-Mach-O object has been provided. However, it does not print the input filename when it should:

Before -
(base) xgupta@archlinux ~/llvm/llvm-project/build (main*) $ llvm-size ~/hello.o --format=darwin
   text	   data	    bss	    dec	    hex	filename
    291	      0	      0	    291	    123	%

After -
(base) xgupta@archlinux ~/llvm/llvm-project/build (main*) $ bin/llvm-size ~/hello.o --format=darwin
   text	   data	    bss	    dec	    hex	filename
    291	      0	      0	    291	    123	/home/xgupta/hello.o

Fix #42316

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D132364