Set PrepareForThinLTO flag when using ThinLTO

The LLVM PassManager has a PrepareForThinLTO flag, which is intended
when compilation occurs in conjunction with linking by ThinLTO. The
flag has two effects:

 * The NameAnonGlobal pass is run after all other passes, which
   ensures that all globals have a name.
 * In optimized builds, a number of late passes (mainly related to
   vectorization and unrolling) are disabled, on the rationale that
   these a) will increase codesize of the intermediate artifacts
   and b) will be run by ThinLTO again anyway.

This patch enables the use of PrepareForThinLTO if Thin or ThinLocal
linking is used.

The background for this change is the CI failure in #49479, which
we assume to be caused by the NameAnonGlobal pass not being run.
As this changes which passes LLVM runs, this might have performance
(or other) impact, so we want to land this separately.

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