Track which modules have debug info variable errors.

Now that we display an error when users try to get variables, but something in the debug info is preventing variables from showing up, track this with a new bool in each module's statistic information named "debugInfoHadVariableErrors".

This patch modifies the code to track when we have variable errors in a module and adds accessors to get/set this value. This value is used in the module statistics and we added a test to verify this value gets set correctly.

Differential Revision: https://reviews.llvm.org/D134508