Introduce namespace langutil in liblangutil directory.

Also:
- Use {}-style list initialisation for SourceLocation construction
- Introduce new system includes
- Changes the API of the Scanner to take source as value (with move) as opposed to as a reference

test/libyul/YulOptimizerTest.cpp

@@ -21,6 +21,9 @@
 
 #include <test/Options.h>
 
+#include <liblangutil/ErrorReporter.h>
+#include <liblangutil/Scanner.h>
+
 #include <libyul/optimiser/BlockFlattener.h>
 #include <libyul/optimiser/VarDeclPropagator.h>
 #include <libyul/optimiser/Disambiguator.h>
@@ -41,12 +44,10 @@
 #include <libyul/optimiser/RedundantAssignEliminator.h>
 #include <libyul/optimiser/Suite.h>
 
-#include <liblangutil/Scanner.h>
 #include <libsolidity/inlineasm/AsmPrinter.h>
 #include <libsolidity/inlineasm/AsmParser.h>
 #include <libsolidity/inlineasm/AsmAnalysis.h>
 #include <libsolidity/interface/SourceReferenceFormatter.h>
-#include <liblangutil/ErrorReporter.h>
 
 #include <boost/test/unit_test.hpp>
 #include <boost/algorithm/string.hpp>
@@ -54,6 +55,7 @@
 #include <fstream>
 
 using namespace dev;
+using namespace langutil;
 using namespace dev::yul;
 using namespace dev::yul::test;
 using namespace dev::solidity;