Add checked information for function declarations to AST reading and writing

[kkjeer]

(See issue #704)
Add the checked scope specifier and written checked scope specifier for function declarations to AST reading and writing, similar to the work done in #716 for compound statements. Follow-up work: move pch tests to a subdirectory (#719).

Testing:

• Modified ast-dump-pch tests to include tests for checked scope information for function declarations.
• Passed manual testing on Windows.
• Passed automated testing on Windows and Linux.

[dtarditi]

Looks good. Thanks!