[lldb] Upstream support for Foundation constant classes

[lldb] Upstream support for Foundation constant classes

Upstream support for NSConstantArray, NSConstantIntegerNumber,
NSConstant{Float,Double}Number and NSConstantDictionary.

We would've upstreamed this earlier but testing it requires
-fno-constant-nsnumber-literals, -fno-constant-nsarray-literals and
-fno-constant-nsdictionary-literals which haven't been upstreamed yet.
As a temporary workaround use the system compiler (xcrun clang) for the
constant variant of the tests.

I'm just upstreaming this. The patch and the tests were all authored by
Fred Riss.

Differential revision: https://reviews.llvm.org/D107660