Fix #23762 - Struct constructor template not callable when forward referenced

[abulgit]

Fixes: #23762
templateDeclarationSemantic resets onemember but computeOneMember() skips the recompute if it already ran, leaving it null. Reset haveComputedOneMember so it recompute

[github-actions]

DMD perf check

Metric	Base	PR	Δ
dmd binary size (stripped)	6.86 MB	6.85 MB	-0.11%

All measurements

Metric	Base	PR	Δ
compile hello.d (instr)	214.9 M	214.9 M	+0.001%
compile hello.d -O -release (instr)	233.2 M	233.2 M	-0.001%
compile Phobos (instr)	5,121.6 M	5,123.5 M	+0.037%
compile Phobos codegen (instr)	1,471.9 M	1,472.0 M	+0.007%
compile vibe.d (instr)	15,128.0 M	15,130.8 M	+0.019%
dmd binary size (stripped)	6.86 MB	6.85 MB	-0.11%
hello binary size (stripped)	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	43.44 MB	43.40 MB	-0.08%
peak RSS (compile Phobos)	617.4 MB	617.4 MB	0.00%
peak RSS (compile vibe.d)	1917 MB	1917 MB	-0.03%
compile dmd itself (wall)	12.6 s	12.9 s	+1.84%
compile hello.d (wall)	63.8 ms	66.5 ms	+4.28%
compile Phobos (wall)	1,572 ms	1,600 ms	+1.76%

_{c5ce142 vs merge-base 4264da7 · about these metrics}

[abulgit]

@dkorpel