[mlir:PDL] Fix a syntax ambiguity in pdl.attribute

pdl.attribute currently has a syntax ambiguity that leads to the incorrect parsing
of pdl.attribute operations with locations that don't also have a constant value. For example:

```
pdl.attribute loc("foo")
```

The above IR is treated as being a pdl.attribute with a constant value containing the location,
`loc("foo")`, which is incorrect. This commit changes the syntax to use `= <constant-value>` to
clearly distinguish when the constant value is present, as opposed to just trying to parse an attribute.

Differential Revision: https://reviews.llvm.org/D124582