Fix compilation on windows with clang

[DirtYiCE]

Currently rapidjson produces many warning messages when compiling it with clang-cl on windows using MSVC 2017 libraries. The problem is that in this case _MSC_VER and __clang__ are both defined, and the headers doesn't handle this case correctly.

The file test/unittest/schematest.cpp had a compile error under gcc with -Werror=effc++ since include/rapidjson/internal/regex.h didn't pop the diagnostic in case of gcc.

[coveralls]

Coverage remained the same at 99.92% when pulling 0fdd804 on tresorit:win-clang-fix into 8022a5f on Tencent:master.

[steveire]

Please also integrate the change in #1231 into this one.