Clang importer: allow subsequent macro definitions to replace earlier ones.
Clang importer: allow subsequent macro definitions to replace earlier ones.

This better matches the behavior we had before enabling the Swift name
lookup tables. Tests will be forthcoming, but this is expected to fix
a failure on Linux.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift