Fixed function params comparison. Updated docs and tests.

Summary: "checkParamTypes" may fail if the the type of some parameter is not canonical. Fixed it by comparing canonical types. And added "getCanonicalType()" and "getCanonicalDecl()" on more places to prevent potential fail.

Reviewers: alexfh

Subscribers: cfe-commits

Patch by Cong Liu!

Differential Revision: http://reviews.llvm.org/D16587

llvm-svn: 259197