[analyzer] Refactor BugTypes and their ownership model.

-In general, don't have the BugReporter deleting BugTypes, BugTypes will eventually become owned by checkers
 and outlive the BugReporter. In the meantime, there will be some leaks since some checkers assume that
 the BugTypes they create will be destroyed by the BugReporter.
-Have BugReporter::EmitBasicReport create BugTypes that are reused if the same name & category strings
 are passed to EmitBasicReport. These BugTypes are owned and destroyed by the BugReporter.
 This allows bugs reported through EmitBasicReport to be coalesced.
-Remove the llvm::FoldingSet<BugReportEquivClass> from BugType and move it into the BugReporter.
 For uniquing BugReportEquivClass also use the BugType* so that we can iterate over all of them using only one set.

llvm-svn: 126272