Revert r196372, "do not warn about unknown pragmas in modes that do not handle them (pr9537)"

It broke clang tests on some hosts with +Asserts. Seems "STDC" clashes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196376 91177308-0d34-0410-b5e6-96231b3b80d8