Changed the ClangASTImporter to store metadata

for each AST context it knows about in a single
object.  This makes it faster to look up the
appropriate ASTImpoter for a given ASTContext
pair and also makes it much easier to delete all
metadata for a given AST context.

In the future, this fix will allow the
ClangASTImporter to propagate completion
information between the metadata for different
AST contexts as its minions move AST objects
around.

llvm-svn: 144835