[lldb] Fix that LLDB doesn't print NaN's sign on Darwin

It seems std::ostringstream ignores NaN signs on Darwin while it prints them on
Linux. This causes that LLDB behaves differently on those platforms which is
both confusing for users and it also means we have to deal with that in our
tests.

This patch manually implements the NaN/Inf printing (which are apparently
implementation defined) to make LLDB print the same thing on all platforms. The
only output difference in practice seems to be that we now print negative NaNs
as `-nan`, but this potentially also changes the output on other systems I
haven't tested this on.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D102845