Cherry-pick 63bc9e443502ab6def2dec0b5ffe64a522f801cc to 12.x release

[kadircet]

Bugzilla Link	49935
Resolution	FIXED
Resolved on	May 03, 2021 16:55
Version	unspecified
OS	All
Depends On	#49249
Blocks	#48246 #48661
CC	@sam-mccall,@tstellar
Fixed by commit(s)	b9b708e 0cbbf06

Extended Description

63bc9e4 is a self-contained change within clangd that provides users with an easy opt-out strategy for a feature we would like to turn on by default on some projects.

The impact radius is small, but it is a quite important blocker for enabling a feature for LLVM developers that we've invested heavily over the past months.

[kadircet]

Note that this requires b9b708e to be cherry-picked first, which is requested in in #49249 .

[sam-mccall]

LGTM if the window is still open...

[tstellar]

Merged: 0cbbf06