The major regression brought by Xcode 12.0 beta 6 (12A8189n) is the deprecation and future removal of the legacy build system.
As a matter of facts, the legacy build system is required to process the output of an external build running on the GCC tool-chain, and provides check-as-you-type and click-to-error features. The new build system remove those features.
So I filed a ticket FB8572278 with the Feedback Assistant at the Developers Tools section.
To my surprise, I received an answer —sort of. I was asked to "provide more details about [my] use case. Specifically, what is a 'CCG toolchain' and why is its use _inherent_ to the legacy build system?".
The loss of critical features due to the removal of the legacy build system, and the nonsense answer from Apple customer service to developers, are a clear invitation to assess alternatives outside the Apple eco-system.
Apple has finally acknowledged that Xcode 12 no longer supports the GCC tool-chain, and that this deprecation may rise other more general issues.
In addition to not officially supporting CCG (with new generation or legacy systems), you may be experiencing more global compatibility issues that we need to look at more closely.
Surprisingly, Apple mentions this critical change nowhere in the Xcode 12 release notes