[tests] Only include stdint.h if we are in freestanding mode.

 - We shouldn't even try to include stdint.h in hosted mode, as the dependency
   on even parsing a platforms stdint.h might fail for some targets.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179723 91177308-0d34-0410-b5e6-96231b3b80d8