[mlir][PDLL] Add support for generating PDL patterns from PDLL at build time

This essentially sets up mlir-pdll to function in a similar manner to mlir-tblgen. Aside
from the boilerplate of configuring CMake and setting up a basic initial test, two new
options are added to mlir-pdll to mirror options provided by tblgen:

* -d
 This option generates a dependency file (i.e. a set of build time dependencies) while
 processing the input file.

* --write-if-changed
 This option only writes to the output file if the data would have changed, which for
 the build system prevents unnecesarry rebuilds if the file was touched but not actually
 changed.

Differential Revision: https://reviews.llvm.org/D124075

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