search for: __llvm_ias__

This seems to be a slightly contentious suggestion, so it seems fitting to bring it up on cfe-dev. Given that -fintegrated-as and -fno-integrated-as are available to the user, and that the integrated assembler can be a bit stringent (as compared to GAS at least, which, for example, supports deprecated syntax), it might be nice to permit the user to detect that an integrated assembler is in use.