This provides a format for externally specifying the include directories
for a source file. The format of the tablegen database is exactly the
same as that for PDLL, namely it includes the absolute source file name and
the set of include directories. The database format is shared to simplify
the infra, and also because the format itself is general enough to share. Even
if we desire to expand in the future to contain the actual compilation command,
nothing there is specific enough that we would need two different formats.
As with PDLL, support for generating the database is added to our mlir_tablegen