In the AST file format, eliminate the CHAINED_METADATA record. Instead,

all AST files have a normal METADATA record that has the same form
regardless of whether we refer to a chained PCH or any other kind of
AST file.

Introduce the IMPORTS record, which describes all of the AST files
that are imported by this AST file, and how (as a module, a PCH file,
etc.). Currently, we emit at most one entry to this record, to support
chained PCH.

llvm-svn: 137869