Add a state variable to the loop hint attribute.

This patch is necessary to support constant expressions which replaces the integer value in the loop hint attribute with an expression. The integer value was also storing the pragma’s state for options like vectorize(enable/disable) and the pragma unroll and nounroll directive. The state variable is introduced to hold the state of those options/pragmas. This moves the validation of the state (keywords) from SemaStmtAttr handler to the loop hint annotation token handler.

Reviewed by Aaron Ballman
[Sanitizer] Make SuppressionContext a singleton class, residing in sanitizer_common.

Convert TSan and LSan to the new interface. More changes will follow:
1) "suppressions" should become a common runtime flag.
2) Code for parsing suppressions file should be moved to SuppressionContext::Init().