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/plugins/texteditor/codestylepool.cpp

@@ -81,7 +81,7 @@ QByteArray CodeStylePoolPrivate::generateUniqueId(const QByteArray &id) const
     }
 
     const QByteArray baseName = id.left(idx);
-    QByteArray newName = baseName.isEmpty() ? "codestyle" : baseName;
+    QByteArray newName = baseName.isEmpty() ? QByteArray("codestyle") : baseName;
     int i = 2;
     while (m_idToCodeStyle.contains(newName))
         newName = baseName + QByteArray::number(i++);