[mlir] Make MLIRContext::getOrLoadDialect(StringRef, TypeID, ...) public

Having this function in a public scope is helpful to register dialects that are
defined at runtime, and thus that need a runtime-defined TypeID.

Also, a similar function in DialectRegistry, insert(TypeID, StringRef, ...), has
a public scope.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D95091