[lld-macho] Support -no_function_starts

Pretty simple code-wise. Also threw in some refactoring:

* Put the functionStartSection under Writer instead of InStruct, since
  it doesn't need to be accessed outside of Writer
* Adjusted the test to put all files under the temp dir instead of at
  the top-level
* Added some CHECK-LABELs to make it clearer where the function starts
  data is

Differential Revision: https://reviews.llvm.org/D99112