Declare __clear_cache.

GCC declares __clear_cache in the gnu modes (-std=gnu++98,
-std=gnu++11), but not in the strict modes (-std=c++98, -std=c++11). This patch
declares it and therefore fixes the build when using one of the strict modes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181785 91177308-0d34-0410-b5e6-96231b3b80d8