d: Fix crash in d/dmd/root/aav.d:127 dmd_aaGetRvalue from DsymbolTable::lookup

Backports patch from upstream dmd mainline for fixing PR110113.

The data being Mem.xrealloc'd contains many Array(T) fields, some of
which have self references in their data.ptr field thanks to the
smallarray optimization used by Array.

Naturally then, the memcpy from old GC data to new retains those self
referenced addresses, and the GC marks the old data as "free". Some time
later GC.malloc will return a pointer to said "free" data. So now we
have two GC references to the same memory. One that is treating the data
as an Array(VarDeclaration) in dmd.escape.escapeByStorage, and the other
as an AA in the symtab of a dmd.dsymbol.ScopeDsymbol.

Fix this memory corruption by not storing the data in a global variable
for reuse.  If there are no more live references, the GC will free it.

	PR d/110113

gcc/d/ChangeLog:

	* dmd/escape.d (checkMutableArguments): Always allocate new buffer for
	computing escapeBy.

gcc/testsuite/ChangeLog:

	* gdc.test/compilable/test23978.d: New test.

Reviewed-on: https://github.com/dlang/dmd/pull/15302