[AST/Sema/libclang] Replace getSourceRange().getBegin() with getLocStart().

 - getSourceRange().getBegin() is about as awesome a pattern as .copy().size().

I already killed the hot paths so this doesn't seem to impact performance on my
tests-of-the-day, but it is a much more sensible (and shorter) pattern.

llvm-svn: 152419