Merge pull request #21 from oven-sh/root/llvm-codegen-attrs

llvm: improve IR attributes and cast flags for better optimization

Merge pull request #21 from oven-sh/root/llvm-codegen-attrs

llvm: improve IR attributes and cast flags for better optimization

Merge pull request #21 from oven-sh/root/llvm-codegen-attrs

llvm: improve IR attributes and cast flags for better optimization

Merge pull request #21 from oven-sh/root/llvm-codegen-attrs

llvm: improve IR attributes and cast flags for better optimization

Merge pull request #21 from oven-sh/root/llvm-codegen-attrs

llvm: improve IR attributes and cast flags for better optimization

ci: name releases by branch so stable and -fast are distinguishable

ci: name releases by branch so stable and -fast are distinguishable

ci: name releases by branch so stable and -fast are distinguishable

ci: name releases by branch so stable and -fast are distinguishable

zig_llvm: emit module summary + EnableSplitLTOUnit on -flto bitcode

Without these, lld rejects the link with "inconsistent LTO Unit splitting"
when mixing zig -flto objects with clang -fwhole-program-vtables bitcode.
Mirrors what clang's BackendUtil does for regular LTO.