Rework InitPreprocessor to use a MacroBuilder class instead of pushing around

std::vectors.

- MacroBuilder wraps a raw_ostream so it can easily write to any buffer
  supported by raw_ostream.
- MacroBuilder's method take Twines for easy string concatenation (this was done
  with sprintf and temporary buffers before).
- Targets still use std::vector as they don't have access to the builder.

llvm-svn: 93051