[LLVM 11 regression] Backport of [MC][ELF] Fix accepting abbreviated form with sh_flags and sh_entsize

[tob2]

Bugzilla Link	48922
Version	11.0
OS	Linux
Blocks	#48289
CC	@tstellar

Extended Description

During the development of LLVM 11, a sensible error was added, but it triggered too often. This was fixed recently (LLVM trunk + 11 backport), cf. Bug #​48201

• https://reviews.llvm.org/D92052

However, I missed a corner case, affecting-real-world code, which has now been fixed:

https://reviews.llvm.org/D94072 – committed to LLVM trunk
as commit 70ea15b

Thus, it makes sense to also backport the follow-up fix to LLVM 11.
Thanks for consideration/merging.

[tob2]

Add blocking to bug 48945 (release-11.1.0) for backporting consideration.

[tob2]

tstellar: Please consider for backporting from LLVM 12 - it is a simple fix, avoiding some breakage.

[tstellar]

mentioned in issue #48289