only stringify the last line in TokenList::readfile() if necessary

[firewave]

lastLine() is only used to check for preprocessor directives. So if it isn't such there's no need to stringify it saving a lot of unnecessary std::string operations.

The intermediate increase comes from not constructing std::string objects on-the-fly for std::string::insert() and simply using the literals.

Testing with -q -Ilib/ -D__GNUC__ lib/valueflow.cpp this saves about 8% of total Ir.

Clang 13 298,836,106 -> 295,950,055 -> 267,545,621
GCC 11 305,058,160 -> 299,749,788 -> 271,347,737

[firewave]

This could probably be simplified by getting rid of lastLine() completely and creating a matcher function instead.