[misc] Show LLVM version on startup

[yuanming-hu]

Concisely describe the proposed feature
Since we are now upgrading from LLVM 8.0.1 to LLVM 10.0.0, it would be good to show the LLVM version when Taichi starts. For example, the startup message could be

[Taichi] <dev mode>, supported archs: [...], commit b62337c9, LLVM 10.0.0, python 3.6.9

Describe the solution you'd like (if any)
Just export macros such as LLVM_VERSION_MAJOR using pybind11. E.g., see get_version_major:

taichi/taichi/python/export_lang.cpp

Line 536 in a493339

m.def("get_version_major", get_version_major);

[archibate]

I think the start up could become too long now...
Maybe we should use a separate command ti diagnose to show valuable system information including but not limited to:

• supported archs (which is removed from startup message in [Misc] Postpone backend detection to prevent possible compatibility issues #1273)
• LLVM version (this issue)
• CUDA version (if available)
• NVIDIA card version (unified memory support? @yuanming-hu)
• OpenGL version & vendor (@archibate)
• OpenGL extensions available (@archibate)
• Metal API version (@k-ye)
• OS dist: Ubuntu, Arch Linux, Win 10 Professional ([gui] [Bug] Fix keyboard event loss problem on specific Win OS version #1162), Mac OS X 10.11 ([mac] [misc] Fix Travis CI: no <filesystem>, use <unistd.h> instead #1161)

[yuanming-hu]

I think the start up could become too long now...
Maybe we should use a separate command ti diagnose to show valuable system information including but not limited to:

• supported archs (which is removed from startup message in [Misc] Postpone backend detection to prevent possible compatibility issues #1273)
• LLVM version (this issue)
• CUDA version (if available)
• NVIDIA card version (unified memory support? @yuanming-hu)
• OpenGL version & vendor (@archibate)
• OpenGL extensions available (@archibate)
• Metal API version (@k-ye)
• OS dist: Ubuntu, Arch Linux, Win 10 Professional ([gui] [Bug] Fix keyboard event loss problem on specific Win OS version #1162), Mac OS X 10.11 ([mac] [misc] Fix Travis CI: no <filesystem>, use <unistd.h> instead #1161)

That's a good point. I think we can show all info that does not need run-time detection in the startup message. Yes, it may seem a bit too long, but it helps us diagnose bug reports much easier (note that it's hard to ask users run ti diagnose before they raise bug reports... We'd better print the info by default.)

[archibate]

But wasn't LLVM version always 10.1 in release mode since it's built-in from CI?

[yuanming-hu]

But wasn't LLVM version always 10.1 in release mode since it's built-in from CI?

Currently we support both llvm 8.0.1 and llvm 10.0.0.

Jenkins (Linux) uses llvm 10.0.0 and other buildbots use 8.0.1.