Allow dso_local on ifunc.

Allow dso_local on ifunc.

It was never fully disallowed. We were rejecting it in the asm parser,
but not in the verifier.

Currently TargetMachine::shouldAssumeDSOLocal returns true for hidden
ifuncs. I considered changing it and moving the check from the asm
parser to the verifier.

The reason for deciding to allow it instead is that all linkers handle
a direct reference just fine. They use the plt address as the address
of the function. In fact doing that means that clang doesn't have the
same bug as gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83782.

This patch then removes the check from the asm parser and updates the
bitcode reader and writer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322378 91177308-0d34-0410-b5e6-96231b3b80d8