[MLIR] Add LocationAttr declarations to ODS

This commit adds location attributes and arrays of locations to OpBase.td.
Using LocationAttrs directly in an operation is useful when the location does
not correspond 1-1 to an operation or value in the IR.  We use this in CIRCT to
store the declaration location of result ports of HWModule operations.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D142148