Remove an unneeded const_cast


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@172370 91177308-0d34-0410-b5e6-96231b3b80d8
Constify argument of Preprocessor::getMacroInfoHistory and propagate to
callers, removing unneeded const_cast


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@172372 91177308-0d34-0410-b5e6-96231b3b80d8
libclang: remove a few const_casts


git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@172373 91177308-0d34-0410-b5e6-96231b3b80d8