ELF: Override DSO definitions when creating __start_* and __stop_* symbols.

Previously we would fail to synthesise a __start_ or __stop_ symbol if
there existed a definition in a DSO. Instead, we would try to link against
the DSO definition. This became possible after D23552 when linking against
lld-produced DSOs but could in principle also occur when linking against
DSOs produced by other linkers.

Not only does it seem more likely that a user would expect the resolved
definition to be local to the executable, but if a __start_ or __stop_
symbol was synthesised by the linker, it is effectively impossible to link
against correctly from a non-PIC executable in a read-only section. Neither
a PLT nor a copy relocation would give us the right semantics here. The only
way the link could succeed is if the executable provided its own synthetic
definition of the symbol.

The fix is to also synthesise the definition if the only definition comes
from a DSO. Since this is what the addOptionalSynthetic function does,
switch to using that function.

Fixes PR30680.

Differential Revision: https://reviews.llvm.org/D25544

llvm-svn: 284168