[LLVM] Include Support/Host.h for declaration of getDefaultTargetTrip…

…le (apache#5268)

In newer versions of LLVM, this header is no longer included by one of
the already included headers in llvm_common.h, so include it explicitly.

src/target/llvm/llvm_common.h

@@ -63,6 +63,7 @@
 #include <llvm/Support/Alignment.h>
 #endif
 #include <llvm/Support/FileSystem.h>
+#include <llvm/Support/Host.h>
 #include <llvm/Support/MemoryBuffer.h>
 #include <llvm/Support/raw_ostream.h>
 #include <llvm/Support/Casting.h>