[Support] Deprecate system_endianness (#68279)

system_endianness() just returns llvm::endianness::native, a
compile-time constant equivalent to std::native in C++20.  This patch
deprecates system_endianness() while replacing all invocations of
system_endianness() with llvm::endianness::native.

While we are at it, this patch replaces
llvm::support::endianness::{big,little} with
llvm::endianness::{big,little} in those statements that happen to call
system_endianness().  It does not go out of its way to replace other
occurrences of llvm::support::endianness::{big,little}.