[BitcodeReader] Infer the correct runtime preemption for GlobalValue

Summary:
To allow bitcode built by old compiler to pass the current verifer,
BitcodeReader needs to auto infer the correct runtime preemption from
linkage and visibility for GlobalValues.

Since llvm-6.0 bitcode already contains the new field but can be
incorrect in some cases, the attribute needs to be recomputed all the
time in BitcodeReader. This will make all the GVs has dso_local marked
correctly if read from bitcode, and it should still allow the verifier
to catch mistakes in optimization passes.

This should fix PR38009.

Reviewers: sfertile, vsk

Reviewed By: vsk

Subscribers: dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D49039

llvm-svn: 336560