Flattener: Attempt to take params from parent allocator in case of bad DWARF

[ajor]

This is needed for running tests in CI, as the older compilers we have there generate bad DWARF with missing template parameters for allocators.

[codecov-commenter]

Codecov Report

Merging #172 (5316ee3) into main (02b7641) will increase coverage by 0.25%.
The diff coverage is 91.02%.

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
+ Coverage   62.58%   62.84%   +0.25%     
==========================================
  Files          88       88              
  Lines        9243     9318      +75     
  Branches     1529     1536       +7     
==========================================
+ Hits         5785     5856      +71     
+ Misses       2641     2640       -1     
- Partials      817      822       +5     

Impacted Files	Coverage Δ
oi/type_graph/Printer.h	100.00% <ø> (ø)
oi/CodeGen.cpp	38.46% <50.00%> (ø)
oi/type_graph/TypeIdentifier.cpp	88.00% <66.66%> (+1.33%)	⬆️
oi/type_graph/Printer.cpp	96.47% <90.00%> (-0.50%)	⬇️
test/test_type_identifier.cpp	92.53% <94.28%> (+1.91%)	⬆️
test/test_codegen.cpp	95.34% <95.65%> (+0.34%)	⬆️
oi/EnumBitset.h	100.00% <100.00%> (ø)

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[ajor]

This causes some problems - see failures on #161. Needs a different approach.

[ajor]

Reworked to have Flattener do most of the work