[ELF] - Use LLVM_FALLTHROUGH in code.

When compiling LLD using GCC 7 it reports warnings like:
"warning: this statement may fall through [-Wimplicit-fallthrough=]"

LLVM has LLVM_FALLTHROUGH macro which can be used to avoid such warnings.
Together with D33036 this patch fixes them.

Differential revision: https://reviews.llvm.org/D32907

llvm-svn: 302666