[flang][Driver] Guard check for pic/pie settings without driver flags (#143530)
The default relocation model for clang depends on the cmake flag
CLANG_DEFAULT_PIE_ON_LINUX. By default it is set to ON, but when it's
OFF, the default relocation model will be "static".
The outcome of the test running clang without any PIC/PIE flags will
depend on the cmake flag, so make sure it only runs when the flag is ON.