[ThinLTO] Default -enable-import-metadata to false

The default value is dependent on `-DLLVM_ENABLE_ASSERTIONS={off,on}` (D22167), which is
error-prone. The few tests checking `!thinlto_src_module` can specify -enable-import-metadata explicitly.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D93959