[llvm-readobj/elf] - Change the behavior of handing DT_SONAME.

[llvm-readobj/elf] - Change the behavior of handing DT_SONAME.

The current situation/behavior is:
1) llvm-readelf doesn't need a string that is specified by `DT_SONAME`.
2) llvm-readobj/elf always tries to read it, even when there is no `DT_SONAME` tag.
3) Because of that both tools reports a warning for many our test cases.

This patch delays getting a SOName string and changes the behavior (llvm-readobj) to
only report a warning when there is a `DT_SONAME` and a string cab't be read.
Warning is not reported for llvm-readelf, as it never tries to dump it.

Differential revision: https://reviews.llvm.org/D89384