I'm using Windows. After updating MSVS19 from v16.4 to v16.6 I faced with a build errors compiling in Debug mode.
It complains on clang-tblgen.exe and llvm-tblgen.exe cmd line args, e.g.
D:\llvm-project\buildvs\Debug\bin>clang-tblgen.exe --help clang-tblgen.exe: Unknown command line argument '--help'. Try: 'clang-tblgen.exe --help' clang-tblgen.exe: Did you mean '--color'?
After some debugging it turned out that VS compiler had a bug. It dynamically creates an object with constexpr ctor in Debug mode. This bug has been fixed in VS2019 v16.5.
But a workaround was implemented for that and everithing works until v16.5 comes.
The workaround became irrelevant since v16.5 and caused build errors.
So I disabled the workaround for VS2019 v16.5 and higher.
Now the build compiles without errors and works correctly.