[analyzer][UninitializedObjectChecker] New flag to ignore records based on it's fields

Based on a suggestion from @george.karpenkov.

In some cases, structs are used as unions with a help of a tag/kind field.
This patch adds a new string flag (a pattern), that is matched against the
fields of a record, and should a match be found, the entire record is ignored.

For more info refer to http://lists.llvm.org/pipermail/cfe-dev/2018-August/058906.html
and to the responses to that, especially http://lists.llvm.org/pipermail/cfe-dev/2018-August/059215.html.

Differential Revision: https://reviews.llvm.org/D51680

llvm-svn: 342220