[include-cleaner] Add a data-structure to capture IWYU pragmas.

PragmaIncludes captures the pragma-based header-mapping information, it is used
in the "Location => Header" step to determine the final spelling header for a
symbol (rather than the header directive).

The structure is by design to be used inside the include-cleaner library
and clangd.

Differential Revision: https://reviews.llvm.org/D136071