[sanitizer] Fix build when scanf interceptors are enabled, and __isoc99_*scanf are not.

Fixes PR16916.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@189015 91177308-0d34-0410-b5e6-96231b3b80d8
[lsan] Add a regression test for building C code.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@189016 91177308-0d34-0410-b5e6-96231b3b80d8