[Clangd] Failed to decode params using 1.x-compatible request message

textDocument/completion sends a TextDocumentPositionParams message in the 2.x
and 3.x. But in 1.x it was instead a TextDocumentPosition with inlined
parameters. This means that the "uri" field is at the top level and not in
textDocument. Because of this, some clients that maintain compability with 1.x
have both uri and textDocument.uri. Clangd, however, early returns in the
presence of anything but 'textDocument' or 'position' which prevents a client
compatible with both 3.x and 1.x to work correctly. If Clangd was a bit more
permissive (no early return), clients implementing all the versions of the
protocol would work.

Patch by Marc-Andre Laperle!

Differential Revision: https://reviews.llvm.org/D32238

llvm-svn: 300991