LSP: Fix positions for code completions

The columns are also zero based. All previously checked language server
that were tested seem to ignore the fact that the completion was
triggered outside of the line when triggering a completion on the last
character in a line. Never the less this issue can easily be reproduced
with other than the go language server when trying to complete code in
the middle of a line.

Fixes: QTCREATORBUG-21325
Change-Id: I1485f041a27b8fe8b85de4e13bac8bbb296dbf69
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/languageclient/languageclientcodeassist.cpp

@@ -267,6 +267,7 @@ TextEditor::IAssistProposal *LanguageClientCompletionAssistProcessor::perform(
     if (!Utils::Text::convertPosition(interface->textDocument(), m_pos, &line, &column))
         return nullptr;
     --line; // line is 0 based in the protocol
+    --column; // column is 0 based in the protocol
     params.setPosition({line, column});
     params.setTextDocument(
                 DocumentUri::fromFileName(Utils::FileName::fromString(interface->fileName())));