[clangd] synthesize fix message when the diagnostic doesn't provide one.

Summary:
Currently if a fix is attached directly to a diagnostic, we repeat the
diagnostic message as the fix message. From eyeballing the top diagnostics,
it seems describing the textual replacement would be much clearer.

e.g.
error: use of undeclared identifier 'goo'; did you mean 'foo'?
action before: use of undeclared identifier 'goo'; did you mean 'foo'?
action after: change 'goo' to 'foo'

Reviewers: ilya-biryukov

Subscribers: klimek, jkorous-apple, ioeric, MaskRay, cfe-commits

Differential Revision: https://reviews.llvm.org/D45069

llvm-svn: 329090