For PR797:

Adjust users of MappedFile to its new non-throwing interface.  Note that in
most cases the lazy step of just throwing after a call to MappedFile was
installed. This was done in the name of incremental changes. Getting rid of
the new throw statements will take adjustment of interfaces and propagation
of errors to higher levels.  Those changes will come in subsequent patches.

llvm-svn: 29817