[libclang] In clang_equalCursors, clear out the "FirstInDeclGroup" part in a declaration cursor

before doing the comparison.

This is because we can't set "FirstInDeclGroup" consistently; for example, when visiting a DeclStmt we will set
it but we don't set it on the result of clang_getCursorDefinition for a reference of the same declaration.

llvm-svn: 171878