Provide method to merge two QQmlJS::SourceLocations

We make have use of SourceLocations to report errors in qmllint/qmlsc. One thing we currently lack there is a way to merge/unite two source locations, which is helpful for reporting errors in sub-parts of sub-trees of the AST, which are spanned by one element's firstSourceLocation and another elements lastSourceLocation.

We should add a merge/unite function to SourceLocation.