Use Clang's FixItHints to correct expressions with "trivial" mistakes (e.g. "." for "->".)

This feature is controlled by an expression command option, a target property and the
SBExpressionOptions setting.  FixIt's are only applied to UserExpressions, not UtilityFunctions,
those you have to get right when you make them.

This is just a first stage.  At present the fixits are applied silently.  The next step
is to tell the user about the applied fixit.

<rdar://problem/25351938>

llvm-svn: 264379