Choose better hash values for std::monostate and valueless variants.

Choose better hash values for std::monostate and valueless variants.

Previously these hashes were 0 and -1 respectively. These seem like common
sentinel values and should be avoided to prevent needless collisions.

This patch changes those values to different arbitrary numbers, which should
hopefully cause less collisions. Because I couldn't help myself I choose the
fundamental constants for gravity and the speed of light.

llvm-svn: 288623