[llvm-readelf] - Introduce describe() helper functions.

These functions can be used to generate strings like
"SHT_?? section with index ?" to describe sections in error/warning messages,
what helps to simplify and generalize them.

Also this allows to isolate the following common code pattern:
`&Sec - &cantFail(Obj->sections()).front();`

Differential revision: https://reviews.llvm.org/D84240