Constant literals are duplicated during the compilation

[sergeykozub]

Constant literals are duplicated during the compilation, this may be in issue if they're large.
Currently, the compilation pipeline creates at least 7 copies of constant literals:

1. HloModuleProto before optimizations;
2. HloModule in the GpuExecutable;
3. HloModuleProto after optimizations in the GpuExecutable;
4. Computation cache in the XlaCompiler;
5. Computation cache in the TF/XLA bridge;
6. LayoutNormalization pass of the HLO pipeline;
7. LHLO IR.

This results in high RAM usage during the compilation. Some of those copies are persisted in the caches.
Note that the protobuf implementation doesn't allow sharing the underlying data, so at least 3 copies of the constants will be alive at the same time (points 1-3 in the list above).

Despite the above, we could still somewhat reduce the RAM usage during the XLA compilation by removing unnecessary copies - i.e. by putting the literal data behind a std::shared_ptr (cannot do this for HloModuleProto, unfortunately).