[ThinLTO] Ensure the TargetLibraryInfo is constructed early enough

Summary:
Without this change, the WholeProgramDevirt pass, which requires the
TargetLibraryInfo, will construct one from the default triple.

Fixes PR38139.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D49278

llvm-svn: 337750