cmGeneratedFileStream: clang-tidy applied to remove redundant `c_str` calls

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

Source/cmExportInstallFileGenerator.cxx

@@ -288,7 +288,7 @@ bool cmExportInstallFileGenerator::GenerateImportFileConfig(
   fileName += this->FileExt;
 
   // Open the output file to generate it.
-  cmGeneratedFileStream exportFileStream(fileName.c_str(), true);
+  cmGeneratedFileStream exportFileStream(fileName, true);
   if (!exportFileStream) {
     std::string se = cmSystemTools::GetLastSystemError();
     std::ostringstream e;