Allow IRInterpreter to deal with non-power-of-2 sized types to support some bitfield accesses.

Summary:
For some bitfield patterns (like the one added by this commit), Clang will
generate non-regular data types like i24 or i48. This patch follows a
pretty naive approach of just bumping the type size to the next power of 2.
DataExtractor know how to deal with weird sizes. The operations on Scalar
do not know how to deal with those types though, so we have to legalize the
size when creating a Scalar.

Reviewers: jingham, clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D51245

llvm-svn: 340880