[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!

llvm-svn: 194236