Merge the "types" and "declarations" blocks in the precompiled header

format, so that we don't end up with multiple declaration and types
blocks. Also, fix a few obscure bugs with PCH loading and generation:

  - If the DeclIDs DenseMap reallocates while we are writing a
    declaration (due to recursively writing other declarations), we
    could end up writing a bad ID to ExternalDefinitions.
  - When loading an ArrayLoc (part of DeclaratorInfo), we need to set
    the size expression to NULL if no size expression was provided.

PCH -> AST rewriting is still partly broken, unfortunately.

llvm-svn: 84293