[ELF] - LTO: do not optimize away symbols accessed from linkerscript.

Previously when BC file had global variable that was accessed from script,
it was optimized away or inlined by IPO. 
In this patch I add symbols at left side of assignment expression as LinkerRedefined,
what prevents optimization for them.

Differential revision: https://reviews.llvm.org/D37059

llvm-svn: 314097