Fix Issue 21120 - Inconsistent mangling of __init symbol

[kinke]

The mangling code previously assumed no identifier would ever start at buffer offset 0. Get rid of that assumption by storing offset+1.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21120	major	Inconsistent mangling of __init symbol

Testing this PR locally

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

dub run digger -- build "stable + dmd#11645"

[thewilsonator]

Please target stable.