Unfortunately when building with LLVM < 15, we're using C++-11,
so the std::optional bits are only available in C++-17.
Even more unfortunately we can't really override this, rather
inherit the fact of using C++-17 when using LLVM > 16.
Thus, we add a slightly messy conditional compilation situation
to allow usage of LDC on older LLVM versions, and with newer
LLVM/libcxx.
Signed-off-by: Ikey Doherty <ikey@serpentos.com>
This reduces the overhead for auto-detecting and setting up an MSVC
toolchain from a very rough 1 second to about 8 milliseconds on my box.
Enabling the auto-detection by default (and so preferring MSVC over the
'internal' toolchain if there's a Visual C++ installation) is now
possible, fixing issues like #3402.
The MSVC setup now consists of the bare minimum - prepending 3
directories to the LIB env var and 1-2 directories to PATH.
1. Main include corresponding to .cpp file, if any.
2. DMD and LDC includes.
3. LLVM includes.
4. System includes.
Also updated a few include guards to match the default format.
