middle-end/66279 - gimplification clobbers shared asm constraints

When the C++ frontend clones a CTOR we do not copy ASM_EXPR constraints
fully as walk_tree does not recurse to TREE_PURPOSE of TREE_LIST nodes.
At this point doing that seems too dangerous so the following instead
avoids gimplification of ASM_EXPRs to clobber the shared constraints
and unshares it there, like it also unshares TREE_VALUE when it
re-writes a "+" output constraint to separate "=" output and matching
input constraint.

	PR middle-end/66279
	* gimplify.cc (gimplify_asm_expr): Copy TREE_PURPOSE before
	rewriting it for "+" processing.

	* g++.dg/pr66279.C: New testcase.