After changing the ``cmGeneratedFileStream`` methods to accept
``std::string const&`` instead of ``const char*`` we don't
need to call ``std::string::c_str`` anymore when passing
a ``std::string`` to a ``cmGeneratedFileStream`` method.

This patch removes all redundant ``std::string::c_str``
calls when passing a string to a ``cmGeneratedFileStream`` method.

It was generated by building CMake with clang-tidy enabled using
the following options:

-DCMAKE_CXX_CLANG_TIDY=/usr/bin/clang-tidy-4.0;-checks=-*,readability-redundant-string-cstr;-fix;-fix-errors