Require GCC 7 and enable C++17

Fix MSVC2019
- result type of conditional expression is ambiguous:
  types 'const char [1]' and 'QByteArray' can be converted
  to multiple common types

Fix MinGW 8.1
- undefined reference to SemanticHighlightNotification::methodName

Fix Utils::transform with std::vector for GCC & MSVC

Unfortunately we cannot get rid of the special variant and optional
implementations, because Apple Clang requires deployment target >= 10.14
for the functions that can throw std::bad_optional_access.

Fixes: QTCREATORBUG-20520
Change-Id: I5c36a70f21f8b0215d2f4fc5c0653a022778d928
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/libs/languageserverprotocol/languagefeatures.cpp

@@ -547,4 +547,8 @@ PrepareRenameResult::PrepareRenameResult(const QJsonValue &val)
     }
 }
 
+SemanticHighlightNotification::SemanticHighlightNotification(const SemanticHighlightingParams &params)
+    : Notification(methodName, params)
+{}
+
 } // namespace LanguageServerProtocol