[MLIR] Register all extensions in CAPI's RegisterEverything

The patch for promised interfaces (a5ef51d7) doesn't register all
extensions in the CAPI's `mlirRegisterAllDialects()` function.  This is
used by the MLIR Python bindings, causing downstream users of the Python
bindings to terminate abruptly.  This patch adds the call to register
all extensions.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D153174