[Target] Improve string interpretation in Target creation

[kparzysz-quic]

• SplitString now preserves escape sequences, but still observes quote characters.
• Added function Interpret that transforms given string according to interpretation rules:
  • outermost quotes are removed (if present),
  • escape sequences inside quotes are preserved verbatim,
  • unquoted escape sequences produce the escaped character (the escape character (\) is removed.
• Interpretation happens every time a value of any type is to be parsed from a string, e.g. Array will first be parsed as an array, then substrings of the input will be parsed as individual elements of that array. In this case, some parts of the initial input will be parsed (and interpreted) twice.
• Implement corresponding stringification functionality.

This new scheme enabled encoding nested arrays of string with any degree of nesting. For example

"-field='\\'foo0\\',\\'bar0,bar1\\'','\\'zing0,zing1\\',\\'fred\\''"

would correspond to the target kind attribute

Array<Array<Array<String>>>("field"))

and have the value

{ { {foo0}, {bar0, bar1} }, { {zing0, zing1}, {fred} } }

[areusch]

implementation looks fine--one question, do we want to enable all of these nested Array<Array<Array>> in target? it feels like we are reaching more for a structured thing.

[kparzysz-quic]

Short answer: no.

This came up when I was working on adding llvm-options to llvm target. That is going to be Array<String>, but I wanted a robust way of embedding arbitrary strings as option values, so that we don't place any restrictions there. So I thought about how to encode string within a string, etc, especially in a context of an array of strings. This is the result.