[BOLT] Please sanitizers

Summary:
In BinaryContext, we had StringRef holding a reference to
an r-value std::string. This triggers clang's address sanitizer
warnings. In MCPlusBuilder we had a left shift overflowing a type,
which is undefined behavior. Similarly, in CallGraph, we had a hash
function shifting a negative value, which is also UB. The last two
triggers the UB sanitizer.

(cherry picked from FBD24661045)