Add a new top level statistic that tracks how many modules have variable errors.

We have a statistic on each module named "debugInfoHadVariableErrors" which tracks when we have debug info, but an error prevented the variables from being displayed. This patch adds a new top level statistic named "totalModuleCountWithVariableErrors" which is a count of the modules that have "debugInfoHadVariableErrors" set to true.

Differential Revision: https://reviews.llvm.org/D138383