[ADT] Don't rely on string literals not being convertible to non-const char* in CachedHashString.

The build was breaking on some platforms because we assumed that
CachedHashString("foo") would match the CachedHashString(StringRef)
constructor rather than the CachedHashString(char*) constructor.

To fix this, provide a CachedHashString(const char*) constructor, and
add a dummy argument to the old CachedHashString(char*) constructor.

llvm-svn: 284892