[include-cleaner] Merge 2 parseIWYUPragma impls in libToolingInclusions

Based on include-cleaner's version, but:

- remove assert that can fail for input `/\<newline>* */`
- assert was also checking the wrong condition: that the prefix *differed* from
  either `//` or from `/*`. Avoid use of strncmp where we can.
- add a comment that the brittleness of the text matching is intentional

Differential Revision: https://reviews.llvm.org/D138780