[mlir:Bytecode] Add shared_ptr<SourceMgr> overloads to allow safe mmap of data

The bytecode reader currently has no mechanism that allows for directly referencing
data from the input buffer safely. This commit adds shared_ptr<SourceMgr> overloads
that provide an explicit and safe way of extending the lifetime of the input. The usage of
these new overloads is adopted in all of our tooling, and is implicitly used in the filename
only parser methods.

Differential Revision: https://reviews.llvm.org/D139366