Investigate whether OpenSSL can be build with clang-cl

[NikitaNikolaenko]

OpenSSL build on Windows is currently using MSVC compiler (cl) since it needs the correct lib AND pdb files generated. If lib case is pretty much self-explanatory, the pdb one is not - it's only the case because the Perl script used to build OpenSSL fails to return a proper exit code when it doesn't find the correct pdb file.

clang-cl should be capable of generating the pdb file: http://blog.llvm.org/2017/08/llvm-on-windows-now-supports-pdb-debug.html