C++: I started to write a C++ formatter.

Ugh - now it turns out that llvm doesn't even tell us where the '#' or the 'define' keyword is; so now we ALSO need to be able to scan for that while processing the "gap" for whitespace, C-comments, and C++-comments, all possibly with embedded backslash-newlines in them. Lets turn this into a (member) function (of TranslationUnit) that "eats" a gap-string_view while searching for a given fixed-string.

Therefore, input: the gap-string_view and optionally a fixed string to look for; output: the number of characters that were processed so far (that is, where the fixed string begins). Throw an exception if the fixed string could not be found.