[Issue 21499] Backref mangling implementation causes OOM in some edge cases

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21499

--- Comment #2 from Iain Buclaw <ibuclaw at gdcproject.org> ---
Bottom of backtrace proves that it is indeed infinite recursion, so the PR
caused a regression.
---
#6127 0x00005555559ed23c in statementSemantic(Statement*, Scope*)
(s=0x7ffff7b165d0, sc=0x7ffff72dc8c0) at
 src/dmd/statementsem.d:132
#6128 0x00005555559e2f4f in Semantic3Visitor::visit(FuncDeclaration*)
(this=0x7fffffffd170, funcdecl=0x7ff
ff7b161b0) at src/dmd/semantic3.d:572
#6129 0x00005555559419ae in FuncDeclaration::accept(Visitor*)
(this=0x7ffff7b161b0, v=0x7fffffffd170) at s
rc/dmd/func.d:2569
#6130 0x00005555559e1a5a in semantic3(Dsymbol*, Scope*) (dsym=0x7ffff7b161b0,
sc=0x7ffff72dc5b0) at src/dm
d/semantic3.d:81
#6131 0x00005555559e1e54 in Semantic3Visitor::visit(Module*)
(this=0x7fffffffd220, mod=0x7ffff7b14d80) at
src/dmd/semantic3.d:194
#6132 0x00005555558af2da in Module::accept(Visitor*) (this=0x7ffff7b14d80,
v=0x7fffffffd220) at src/dmd/dm
odule.d:1485
#6133 0x00005555559e1a5a in semantic3(Dsymbol*, Scope*) (dsym=0x7ffff7b14d80,
sc=0x0) at src/dmd/semantic3
.d:81
#6134 0x000055555599999e in dmd.mars.tryMain(ulong, const(char)**, ref
dmd.globals.Param) (params=..., arg
v=0x7fffffffdd68, argc=2) at src/dmd/mars.d:579
#6135 0x000055555599b11d in D main (_param_0=...) at src/dmd/mars.d:972

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