Use consistent spacing before custom directives for op and attr/type assemblyFormat.

Currently, assemblyFormat `custom<A>($a) custom<B>($b)` has different spacing
if used for Ops vs Attrs/Types. Ops insert a space if needed before the custom directive,
while attributes and types do not.

This leads to the following two patterns in attributes / types:

```
# 1. Whitespace literal
let assemblyFormat = "... ` ` custom<A>($a)"

# 2. Custom printer code includes spacing
void printB(...) {
  printer << ' ' << b;
}
```

Moving this spacing into the generated code allows for some cleanup in mlir and
improves the consistency of custom directives.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D138235