[Coverage] Ensure that coverage mapping data has an expected alignment in 'covmapping' files.

Coverage mapping data is organized in a sequence of blocks, each of which is expected
to be aligned by 8 bytes. This feature is used when reading those blocks, see
VersionedCovMapFuncRecordReader::readFunctionRecords(). If a misaligned covearge
mapping data has more than one block, it causes llvm-cov to fail.

Differential Revision: http://reviews.llvm.org/D20285

llvm-svn: 269887