Improve build time of register_symbols.cpp without compiler hacks (#14911)

Improve build time of register_symbols.cpp without compiler hacks (#14911)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14911

In optimized modes the compiler tries to inline all the
`unordered_map::operator[]` calls, creating a massive amount of code
which takes several minutes to optimize. Instead, create a table of
PODs and populate the maps using a simple loop.

Reviewed By: soumith, luciang

Differential Revision: D13382948

fbshipit-source-id: b6752921e0f7213595d26b39e4397f6a3897960b