Added code that will allow completely customizable prompts for use in

replacing the "(lldb)" prompt, the "frame #1..." displays when doing
stack backtracing and the "thread #1....". This will allow you to see 
exactly the information that you want to see where you want to see it.
This currently isn't hookup up to the prompts yet, but it will be soon.

So what is the format of the prompts? Prompts can contain variables that
have access to the current program state. Variables are text that appears
in between a prefix of "${" and ends with a "}". Some of the interesting
variables include:

// The frame index (0, 1, 2, 3...)
${frame.index}

// common frame registers with generic names
${frame.pc}
${frame.sp}
${frame.fp}
${frame.ra}
${frame.flags}

// Access to any frame registers by name where REGNAME is any register name:
${frame.reg.REGNAME}

// The current compile unit file where the frame is located
${file.basename}
${file.fullpath}

// Function information
${function.name}
${function.pc-offset}

// Process info
${process.file.basename}
${process.file.fullpath}
${process.id}
${process.name}

// Thread info
${thread.id}
${thread.index}
${thread.name}
${thread.queue}
${thread.stop-reason}

// Target information
${target.arch}

// The current module for the current frame (the shared library or executable
// that contains the current frame PC value):
${module.file.basename}
${module.file.fullpath}

// Access to the line entry for where the current frame is when your thread
// is stopped:
${line.file.basename}
${line.file.fullpath}
${line.number}
${line.start-addr}
${line.end-addr}

Many times the information that you might have in your prompt might not be
available and you won't want it to print out if it isn't valid. To take care
of this you can enclose everything that must resolve into a scope. A scope
is starts with '{' and ends with '}'. For example in order to only display
the current file and line number when the information is available the format
would be:

"{ at {$line.file.basename}:${line.number}}"

Broken down this is:

start the scope: "{"

format whose content will only be displayed if all information is available:
        "at {$line.file.basename}:${line.number}"

end the scope: "}"

We currently can represent the infomration we see when stopped at a frame:

frame #0: 0x0000000100000e85 a.out`main + 4 at test.c:19

with the following format:

"frame #${frame.index}: ${frame.pc} {${module.file.basename}`}{${function.name}{${function.pc-offset}}{ at ${line.file.basename}:${line.number}}\n"

This breaks down to always print:

        "frame #${frame.index}: ${frame.pc} "

only print the module followed by a tick if we have a valid module:

        "{${module.file.basename}`}"
        
print the function name with optional offset:
        "{${function.name}{${function.pc-offset}}"

print the line info if it is available:
        
        "{ at ${line.file.basename}:${line.number}}"

then finish off with a newline:

        "\n"

Notice you can also put newlines ("\n") and tabs and everything else you
are used to putting in a format string when desensitized with the \ character.

Cleaned up some of the user settings controller subclasses. All of them 
do not have any global settings variables and were all implementing stubs
for the get/set global settings variable. Now there is a default version
in UserSettingsController that will do nothing.

llvm-svn: 114306