Clang: Support second translation unit

A TranslationUnit is owned by TranslationUnits now. TranslationUnits
allows to add another TranslationUnit and to update/query the recently
and previously parsed translation unit.

This does not change any behavior yet.

Change-Id: I8a2f0cc05d3e51bf739dd5d7c4da14b54147f3ab
Reviewed-by: David Schulz <david.schulz@qt.io>

src/tools/clangbackend/ipcsource/clangexceptions.cpp

@@ -94,4 +94,11 @@ DocumentProcessorDoesNotExist::DocumentProcessorDoesNotExist(const Utf8String &f
            + Utf8StringLiteral("' does not exist!");
 }
 
+TranslationUnitDoesNotExist::TranslationUnitDoesNotExist(const Utf8String &filePath)
+{
+    m_info += Utf8StringLiteral("TranslationUnit for file '")
+            + filePath
+            + Utf8StringLiteral("' does not exist.");
+}
+
 } // namespace ClangBackEnd