clangd CLI usability improvements

[sam-mccall]

Bugzilla Link	42752
Resolution	FIXED
Resolved on	Jul 25, 2019 08:06
Version	unspecified
OS	Linux
Blocks	#41819
CC	@zmodem
Fixed by commit(s)	r366811, r366880, r366900, r366991, r366992

Extended Description

Some unintrusive changes to ClangdMain.cpp to improve usability.
Would be great to slip these into the llvm-9 release.

366811 - Log version, cwd, args, and transport on startup. NFC
366880 - Reformat use of cl::opt: use unqualified name and don't bin-pack attributes. NFC (needed to avoid merge conflicts on next patch)
366900 - Add categories to help options, and only show clangd options.
366991 - Also accept flags from CLANGD_FLAGS variable.
366992 - Provide help text to users who run clangd in a terminal.

[sam-mccall]

@​hans these are very safe (window-dressing), but if this isn't appropriate at this stage of the release feel free to drop.

[zmodem]

Merged them all in r367025.