Fix even more Clang warnings.

This patch disables shift-sign-overflow warnings for now. It also
fixes most -Wfloat-equal warnings and -Wextra-semi warnings.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@343438 91177308-0d34-0410-b5e6-96231b3b80d8