[LLD][ELF] - Handle SHT_GROUP more carefully. NFCI.

This patch solves 2 problems:
1) It adds a test to check the line below:
https://github.com/llvm-mirror/lld/blob/master/ELF/InputFiles.cpp#L334
Test case contains SHT_GROUP section with a broken (0xFF) flag.

2) The patch fixes the case when we silently accepted such broken groups
in the case when there were no other objects with the same group signature.

llvm-svn: 339765