[llvm] Remove out-of-date fixme from SmallVectorMemoryBuffer

This fixme first appeared in the codebase with the introduction of `ObjectMemoryBuffer` in rG93de2a12a36feaf1e6d8ff28c76db9b6cda4e844, but the constructor appears to never have been templated.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D115044