Add support for "dynamic values" for C++ classes.  This currently only works for "frame var" and for the
expressions that are simple enough to get passed to the "frame var" underpinnings.  The parser code will
have to be changed to also query for the dynamic types & offsets as it is looking up variables.

The behavior of "frame var" is controlled in two ways.  You can pass "-d {true/false} to the frame var
command to get the dynamic or static value of the variables you are printing.

There's also a general setting:

target.prefer-dynamic-value (boolean) = 'true'

which is consulted if you call "frame var" without supplying a value for the -d option.

llvm-svn: 129623