Include what you use in LanaiMCCodeEmitter.cpp

LanaiMCCodeEmitter.cpp was not using any APIs from Lanai.h, and was only
including it for transitive dependencies.  Doing so is problematic from
include-what-you-use perspective, but it is also a layering issue (it
creates a dependency cycle between the primary Lanai target library and
the MCTargetDesc library).

llvm-svn: 362394