[dmd-cxx] Backport add toTypeFunction() to catch bad casts

[ibuclaw]

Backport from #6643. When running D2 testsuite with ubsan build, it was throwing errors in FuncDeclaration::semantic.

e.g:

fail_compilation/imports/a13311.d:8:5: error: undefined identifier ‘PieceTree’
func.c:557: runtime error: load of value 217, which is not a valid value for type 'bool'
func.c:562: runtime error: load of value 16384, which is not a valid value for type 'TRUST'
func.c:564: runtime error: load of value 16384, which is not a valid value for type 'TRUST'
func.c:565: runtime error: load of value 16384, which is not a valid value for type 'TRUST'
func.c:566: runtime error: load of value 16384, which is not a valid value for type 'TRUST'
func.c:583: runtime error: load of value 144, which is not a valid value for type 'bool'

This was because of blind casting from TypeError to TypeFunction, which was fixed in master by this PR.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "dmd-cxx + dmd#9054"