This patch adds an IsText parameter to the following getBufferForFile,
getBufferForFileImpl. We introduce a new virtual function
openFileForReadBinary which defaults to openFileForRead except in
RealFileSystem which uses the OF_None flag instead of OF_Text.
The default is set to OF_Text instead of OF_None, this change in value
does not affect any other platforms other than z/OS. Setting this
parameter correctly is required to open files on z/OS in the correct
encoding. The IsText parameter is based on the context of where we open
files, for example, in the ASTReader, HeaderMap requires that files
always be opened in binary even though they might be tagged as text.
