[ELF] - Fix for: Bug 30521 - lld exits non-zero return value linking a library with no entry symbol

PR30521 was about linking shared library. After r282295 code when linking -shared produced
"entry symbol not found" warning, what in combination with --fatal-errors failed linkage.

Patch fixes logic (and adds testcases) to follow next rules:

1) If entry was specified and not found report warning.
2) If entry was not specified then:
 a) Emit warning if not -shared.
 b) Do not emit warning if -shared.

Differential revision: https://reviews.llvm.org/D24913

llvm-svn: 282427