clang reports many warnings

[antoninbas]

with clang++-3.6:
for example:

  CXX      tools/ir-generator/ir-generator.o
In file included from ../tools/ir-generator/ir-generator.ypp:18:
In file included from ../tools/ir-generator/irclass.h:14:
../tools/ir-generator/type.h:24:21: warning: 'toString' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
    virtual cstring toString() const = 0;
                    ^
../lib/source_file.h:198:21: note: overridden virtual function is here
    virtual cstring toString() const = 0;