MLIR loc interpreted as pdl.attribute value

[henrytwo]

Consider the following PDL pattern, where we have an attribute with a known type:

pdl.pattern : benefit(1)  {
  %0 = pdl.type : i32
  %1 = pdl.attribute : %0 loc("-":15:12)
  
  %2 = pdl.operation "C" {"Foo" = %1}
  pdl.rewrite {
    pdl.erase %2
  } 
}

As the custom PDL syntax is currently defined, the location will be inadvertently interpreted as the value for the attribute.

operation ::= `pdl.attribute` (`:` $type^)? ($value^)? attr-dict-with-keyword

This results in the following error when passed through mlir-opt on LLVM version 14.0.0:

<stdin>:3:8: error: 'pdl.attribute' op expected only one of [`type`, `value`] to be set
  %1 = pdl.attribute : %0 loc("-":15:12)
       ^
<stdin>:3:8: note: see current operation: %1 = "pdl.attribute"(%0) {value = loc("-":15:12)} : (!pdl.type) -> !pdl.attribute

The above MLIR originated from a mlir::Operation in C++ and was printed using the following flags (irrelevant locations omitted for brevity):

mlir::OpPrintingFlags().useLocalScope().enableDebugInfo()

This looks like a bug (or at least an issue with the op syntax), since this MLIR does not make it through a full round trip after being printed.