[analyzer] Add IdenticalExprChecker, to find copy-pasted code.

This syntactic checker looks for expressions on both sides of comparison
operators that are structurally the same. As a special case, the
floating-point idiom "x != x" for "isnan(x)" is left alone.

Currently this only checks comparison operators, but in the future we could
extend this to include logical operators or chained if-conditionals.

Checker by Per Viberg!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@194236 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed