The toolchain has traditionally been buildable with clang, cl (MSVC),
and gcc. `__has_feature` is a clang-specific directive, ensure that we
are portable to other compilers by providing a definition of the macro
incase it is not available (which also includes older clang releases).
Fixes a couple of compiler warnings that occur frequently when building the compiler:
- Copy the nullability annotation definitions from `Visibility.h` to `BridgedSwiftObject.h` and wrap all code that contains nullability annotations in `SWIFT_BEGIN_NULLABILITY_ANNOTATIONS` and `SWIFT_END_NULLABILITY_ANNOTATIONS` (supressing the warning `type nullability specifier '_Nullable' is a Clang extension [-Wnullability-extension]`)
- Suppress warnings about using `$` (mangling prefix) as an identifier using pragmas (supressing the warning `'$' in identifier [-Wdollar-in-identifier-extension]`)
- Change the macro condition of `SWIFT_NODISCARD` from `__cplusplus >= 201402l` (which checked for >= C++14) to `__cplusplus > 201402l`. This appears to have been a copy-paste error from `LLVM_NODISCARD` (supressing the warning `use of the 'nodiscard' attribute is a C++17 extension [-Wc++17-extensions]`)
This is the initial version of a buildable SIL definition in libswift.
It defines an initial set of SIL classes, like Function, BasicBlock, Instruction, Argument, and a few instruction classes.
The interface between C++ and SIL is a bridging layer, implemented in C.
It contains all the required bridging data structures used to access various SIL data structures.
