Add checked information for compound statements to AST reading/writing

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#716

---

(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.