LDC 1.27.0-beta3

[kinke]

Glad to announce the third beta for LDC 1.27. Some noteworthy 
changes since beta2:

* Frontend and druntime/Phobos upgraded to todays's DMD stable.
* LLVM for prebuilt packages bumped to v12.0.1.
* New LDC-specific language addition: `__traits(initSymbol, 
<aggregate type>)`
* Fix missed RVO opportunity.

Full release log and downloads: 
https://github.com/ldc-developers/ldc/releases/tag/v1.27.0-beta3

Please help test, and thanks to all contributors & sponsors!