Directly write to a -Map file.

Previous code had a bug that if the program exits with an assert() or
fail() before the control reaches end of writeMapFile(), it leaves a
temporary file, because FileRemover's dtor isn't called in that case.

I could fix that by removeFileOnSignal() and other functions, but
I think we can simply write to the result file directly. I think
that is straightforward and easy to understand.

Additionally, that allows something like `-Map /dev/null` or a bash
hack such as `-Map >(grep symbol-im-looking-for)`. Previously,
that kind of things didn't work.

Differential Revision: https://reviews.llvm.org/D28714

llvm-svn: 292041