Clang: fix utf8 related column numbers

Use new clang_getFileContents to efficiently convert
utf8 byte offsets from line start to column numbers.
Also provide simplier backwards convertion to pass
resulting utf8 offset to clang.

Task-number: QTCREATORBUG-16941
Change-Id: If0e58fe01ad3e281b7e952e972b9e86f6e75aadb
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/tools/clangbackend/source/sourcerange.h

@@ -40,7 +40,7 @@ class SourceRange
 
 public:
     SourceRange();
-    SourceRange(CXSourceRange cxSourceRange);
+    SourceRange(CXTranslationUnit cxTranslationUnit, CXSourceRange cxSourceRange);
     SourceRange(const SourceLocation &start, const SourceLocation &end);
 
     bool isNull() const;
@@ -58,6 +58,7 @@ public:
 
 private:
     CXSourceRange cxSourceRange;
+    CXTranslationUnit cxTranslationUnit = nullptr;
 };
 
 bool operator==(const SourceRange &first, const SourceRange &second);