[LLDB][formatters] Add formatter for libc++'s std::span

This patch adds a libcxx formatter for std::span. The
implementation is based on the libcxx formatter for
std::vector. The main difference is the fact that
std::span conditionally has a __size member based
on whether it has a static or dynamic extent.

Example output of formatted span:

(std::span<const int, 18446744073709551615>) $0 = size=6 {
  [0] = 0
  [1] = 1
  [2] = 2
  [3] = 3
  [4] = 4
  [5] = 5
}
The second template parameter here is actually std::dynamic_extent,
but the type declaration we get back from the TypeSystemClang is the
actual value (which in this case is (size_t)-1). This is consistent
with diagnostics from clang, which doesn't desugar this value either.
E.g.,:

span.cpp:30:31: error: implicit instantiation of undefined template
    'Undefined<std::span<int, 18446744073709551615>>'
Testing:

Added API-tests

Confirmed manually using LLDB cli that printing spans works in various scenarios

Patch by Michael Buch!

Differential Revision: https://reviews.llvm.org/D127481