c++, gimplify: Implement C++26 P2795R5 - Erroneous behavior for uninitialized reads...

c++, gimplify: Implement C++26 P2795R5 - Erroneous behavior for uninitialized reads: Adjust 'libgomp.c++/{target-flex-101.C,target-std__flat_map-concurrent.C,target-std__flat_multimap-concurrent.C}' [PR114457, PR122268, PR120450]

With commit r16-4212-gf256a13f8aed833fe964a2ba541b7b30ad9b4a76
"c++, gimplify: Implement C++26 P2795R5 - Erroneous behavior for uninitialized reads [PR114457]",
we acquired:

    {+FAIL: libgomp.c++/target-flex-101.C (internal compiler error: in assign_temp, at function.cc:990)+}
    [-PASS:-]{+FAIL:+} libgomp.c++/target-flex-101.C (test for excess errors)
    [-PASS:-]{+UNRESOLVED:+} libgomp.c++/target-flex-101.C [-execution test-]{+compilation failed to produce executable+}

... for GCN, nvptx offloading compilation, and on the other hand:

    [-XFAIL:-]{+XPASS:+} libgomp.c++/target-std__flat_map-concurrent.C (internal compiler error[-: in assign_temp, at function.cc:990)-]
    [-XFAIL:-]{+XPASS:+} libgomp.c++/target-std__flat_map-concurrent.C (test for excess errors)
    [-UNRESOLVED:-]{+PASS:+} libgomp.c++/target-std__flat_map-concurrent.C [-compilation failed to produce executable-]{+execution test+}

    [-XFAIL:-]{+XPASS:+} libgomp.c++/target-std__flat_multimap-concurrent.C (internal compiler error[-: in assign_temp, at function.cc:990)-]
    [-XFAIL:-]{+XPASS:+} libgomp.c++/target-std__flat_multimap-concurrent.C (test for excess errors)
    [-UNRESOLVED:-]{+PASS:+} libgomp.c++/target-std__flat_multimap-concurrent.C [-compilation failed to produce executable-]{+execution test+}

... for GCN offloading compilation (already PASSed for nvptx).

Note that these test cases explicitly use '-std=c++23', so don't undergo the
new C++26 P2795R5 functionality.  Yet, comparing before vs. after that commit,
in the 'gimple' dumps (that is, early host compilation), there are a lot of
changes where 'gimple_assign <constructor, [...], {CLOBBER(bob)}, NULL, NULL>'s
and relatedly 'gimple_bind's newly appear/no longer appear elsewhere.  This
leads to correspondingly different code at the beginning of offloading
compilation.  Why/how that now ('libgomp.c++/target-flex-101.C') vs. before
('libgomp.c++/{target-std__flat_map-concurrent.C,target-std__flat_multimap-concurrent.C}')
translates into 'expand' ICEs, I can't tell.

	PR c++/114457
	PR c++/122268
	PR c++/120450
	libgomp/
	* testsuite/libgomp.c++/target-flex-101.C: XFAIL GCN, nvptx
	offloading compilation.
	* testsuite/libgomp.c++/target-std__flat_map-concurrent.C:
	Un-XFAIL GCN offloading compilation.
	* testsuite/libgomp.c++/target-std__flat_multimap-concurrent.C:
	Likewise.