[build.py] Add `VCINSTALLDIR` to default variables

[aleksandr.urakov]

Summary

On my system, clang-cl can't compile tests containing char16_t and char32_t types without this environment variable defined. This fix makes the tests to pass, but I'm not sure about the such solution. What do you think about it?

[labath]

I don't think this should be necessary. Based on my experiments, the fact whether clang-cl defines char32_t depends on the version of visual studio it is emulating (-fms-compatibility-version). For instance I can do this:

$ cat /tmp/a.cc 
char32_t XX;
$ bin/clang-cl /tmp/a.cc /GS- /GR- /c -fms-compatibility-version=18
/tmp/a.cc(1,1) :  error: unknown type name 'char32_t'
char32_t XX;
^
1 error generated.
$ bin/clang-cl /tmp/a.cc /GS- /GR- /c -fms-compatibility-version=19
$ bin/clang-cl /tmp/a.cc /GS- /GR- /c

So, what I suspect is happening is that clang uses %VCINSTALLDIR% to detect the MSVC version, and then sets the compatibility based on that. If the detected version is high enough, char32_t will be automatically defined. The interesting part is that, for me, the default version seems to be high enough that even without any special setting char32_t "just works". However, I'm on linux, so the differences could be due to that.

So I think this is good news. I hope all that's needed here is to get build.py to add the -fms-compatibility thingy and things should work. Until we actually need to vary this version, I think we can just unconditionally add that flag to all clang-cl invocations in build.py.

[aleksandr.urakov]

I've checked your solution, and it's worked for me too! I think that your solution is better because it is more straight-forward. Here is the updated patch.

[labath]

This looks good to me, but let's wait for zachary too.

In D57809#1386883, @aleksandr.urakov wrote:

I've checked your solution, and it's worked for me too! I think that your solution is better because it is more straight-forward. Here is the updated patch.

It also has a higher chance of behaving identically on different systems.

[aleksandr.urakov]

Zachary, can you take a look? please?

lldb/trunk/lit/helper/build.py

def _get_compilation_command(self, source, obj):
                 args.append('-flto=thin')
         if self.nodefaultlib:
             args.append('/GS-')
             args.append('/GR-')
         args.append('/Z7')
         if self.toolchain_type == 'clang-cl':
             args.append('-Xclang')
             args.append('-fkeep-static-consts')
+            args.append('-fms-compatibility-version=19')
         args.append('/c')
 
         args.append('/Fo' + obj)
         args.append(source)
 
         return ('compiling', [source], obj,
                 self.compile_env,
                 args)