[NFC] Remove doxygen brief tag from BasicBlock.h

Summary: Documentation is built using the AutoBrief configuration option in docs/doxygen.cfg.in.  This change removes the redundant brief tags from BasicBlock.h.   I'm happy to write a sed script and remove all \brief tags as a separate commit later.

Reviewers: aprantl

Reviewed By: aprantl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D45708

llvm-svn: 330242