Implement __builtin_LINE() et. al. to support source location capture.
This patch implements the source location builtins __builtin_LINE(), builtin_FUNCTION(), builtin_FILE() and __builtin_COLUMN(). These builtins are needed to implement [std::experimental::source_location`](https://rawgit.com/cplusplus/fundamentals-ts/v2/main.html#reflection.src_loc.creation).
With the exception of __builtin_COLUMN, GCC also implements these builtins, and Clangs behavior is intended to match as closely as possible.
Reviewers: rsmith, joerg, aaron.ballman, bogner, majnemer, shafik, martong
Reviewed By: rsmith
Subscribers: rnkovacs, loskutov, riccibruno, mgorny, kunitoki, alexr, majnemer, hfinkel, cfe-commits
Differential Revision: https://reviews.llvm.org/D37035