Remove the Python dependency for generating convert builtins, aligning
with how other builtins are defined.
In addition, our downstream target relies on this PR to override convert
implementations.
llvm-diff shows no changes to all bitcodes:
amdgcn--amdhsa.bc, barts-r600--.bc, cayman-r600--.bc, cedar-r600--.bc,
clspv64--.bc, clspv--.bc, cypress-r600--.bc, nvptx64--.bc,
nvptx64--nvidiacl.bc, nvptx--.bc, nvptx--nvidiacl.bc, tahiti-amdgcn--.bc
and tahiti-amdgcn-mesa-mesa3d.bc.
776f5933c9