Create a _StringRepresentation struct to standardize internal testing
on. Internalize much of _StringGuts, except for some SPI hacks, and
update tests to use _StringRepresentation.
The initial version of the debugger testing transform instruments
assignments in a way that allows the debugger to sanity-check its
expression evaluator.
Given an assignment expression of the form:
```
a = b
```
The transform rewrites the relevant bits of the AST to look like this:
```
{ () -> () in
a = b
checkExpect("a", stringForPrintObject(a))
}()
```
The purpose of the rewrite is to make it easier to exercise the
debugger's expression evaluator in new contexts. This can be automated
by having the debugger set a breakpoint on checkExpect, running `expr
$Varname`, and comparing the result to the expected value generated by
the runtime.
While the initial version of this testing transform only supports
instrumenting assignments, it should be simple to teach it to do more
interesting rewrites.
There's a driver script available in SWIFT_BIN_DIR/lldb-check-expect to
simplfiy the process of launching and testing instrumented programs.
rdar://36032055
When pretty-printing objects, attempt to expand & print objects which
have the `.class` display style even if they do not have any instance
variables. The pretty-printer will still bail out if the object does not
conform to CustomReflectable.
This is enough to teach the pretty-printer to format bridged NSStrings.
rdar://36843869
