Support `alias` attribute on functions

[dkolsen-pgi]

Test program:

void herp_derp() {}
[[gnu::alias("_Z9herp_derpv")]] void frobble();
int main() {
    frobble();
}

Clang and GCC compile and link this successfully. But ClangIR doesn't process the alias attribute and fails to link:

$ cir alias.cpp
/usr/bin/ld: /tmp/alias-7b2e15.o: in function `main':
/local/home/dolsen/upstream/llvm-project/test/alias.cpp:4: undefined reference to `frobble()'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

The bug can be seen in the LLVM IR. Clang creates an alias directive for frobble:

@_Z7frobblev = dso_local alias void (), ptr @_Z9herp_derpv

while ClangIR merely declares the function:

declare !dbg !8 void @_Z7frobblev()

[bcardosolopes]

I just hit this while working through an issue, so I might as well fix it while there.

[bcardosolopes]

While working at this, I noticed that LLVM IR dialect GlobalOp does not yet support alias, so we cannot yet lower this information down. Let me file another issue for that.

For now, I'm going to emit a global but not yet set the alias, won't be much better than what we currently have but it's a first step - later on we can just set the alias when MLIR support comes.