swift234

[json] Added a basic JSON serialization class in swiftBasic.

The swift::json namespace now contains an Output class, which largely mirrors
llvm::yaml::Output. It takes the same approach where there are various traits
structs which dictate how a particular type is output in JSON. (This is separate
from llvm::yaml because, while all JSON is valid YAML, not all YAML is valid
JSON, and customization on how scalar types are output as JSON is necessary.)

Unlike llvm::yaml, there is no equivalent Input class. Since JSON is valid YAML,
llvm::yaml::Input can be used instead.

At some point, the traits structs could likely be merged with llvm::yaml (with
some ability to customize how scalars are output if it's outputting JSON instead
of YAML), but this provides enough of a starting point to allow the driver to
generate parseable output in JSON format.

Part of <rdar://problem/15958329>.

Swift SVN r20870