Lex: add a callback for `#pragma mark`

Lex: add a callback for `#pragma mark`

Allow a preprocessor observer to be notified of mark pragmas.  Although
this does not impact code generation in any way, it is useful for other
clients, such as clangd, to be able to identify any marked regions.

Reviewed By: dgoldman

Differential Revision: https://reviews.llvm.org/D105368