In this testcase, we have multiple declarations of malloc.  The one in
the "system header" has a non-throwing exception specification, while
the one in user code has none.  By default we allow this mismatch when
the old declaration is in a system header, but we mean to prefer the
type of the user declaration.  In this testcase that was breaking
because the old declaration had attributes added by the compiler, and
the code at the bottom of merge_types decided to throw away the work
earlier in the function to build up a new FUNCTION_TYPE in t1 because
t2 had the right attributes.  This is a C++17 regression.

Fixed by limiting the attribute checking code to the case where we
aren't building up a new type in t1.

Tested x86_64-pc-linux-gnu, applying to trunk.