[sanitizer] NFC: add static_assert to confirm that we use reasonable ByteMap type

Summary: If bots work we can replace #ifs with template specialization by TwoLevelByteMapSize1.

There is known users of TwoLevelByteMap with TwoLevelByteMapSize1 equal 8,
and users of FlatByteMap with TwoLevelByteMapSize1 equal 2.

Reviewers: eugenis

Subscribers: kubamracek, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D61200

llvm-svn: 359364