[MC][ELF] Fix accepting abbreviated form with sh_flags and sh_entsize
- D73999 / commit 75af9da755721123e62b45cd0bc0c5e688a9722a added for LLVM 11 a check that sh_flags and sh_entsize (and sh_type) changes are an error, in line with GNU assembler.
- D92052 / commit 1deff4009e0ae661b03682901bf6932297ce7ea1 permitted the abbreviated form which many assemblers accept and GCC generates: while the first .section contains the flags and entsize, subsequent sections simply contain the name without repeating entsize or flags.
However, the latter patch missed in the check that some flags are automatically set, e.g. '.rodata." implies ELF::SHF_ALLOC.
Reviewed By: jhenderson
Differential Revision: https://reviews.llvm.org/D94072