Add checked information for compound statements to AST reading/writing

[kkjeer]

(Issue #419): Modify ASTWriterStmt and ASTReaderStmt to write/read the checked scope specifier and written checked scope specifier for compound statements.

Future work: add checked information for function declarations to ASTWriterDecl and ASTReaderDecl (see issue #704).

Testing:

• Added pch testing that checks the AST output of compound statements with checked information (using the -ast-dump-all option to dump the AST of the precompiled header).
• Passed local testing on Windows.
• Passed automated testing on Windows/Linux.

[dtarditi]

Looks good, thanks! The pch tests probably need to moved to a subdirectory. That can be done separately.