Don't try to install the __[u]int128_t identifier if it is already installed by PCHReader.

Currently, adding it to visible decls of a PCH'ed translation unit has no effect because
adding visible decls before deserialization has no effect (the decls won't be visible).
This will be fixed in a future commit; then it will force deserialization of visible decls, so avoid pointlessly installing it.

llvm-svn: 107595