[lld][linkerscript] Fix handling of DEFINED.

Current implementation did not check that symbols is actually defined. Only checked for presence.  GNU ld documentation says,

"Return 1 if symbol is in the linker global symbol table and is defined before the statement using DEFINED in the script, otherwise return 0."

https://sourceware.org/binutils/docs/ld/Builtin-Functions.html#Builtin-Functions

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D83758