Extend llvmcall with support for entering declarations.

[maleadt]

This PR adds support to llvmcall for inserting required declarations in the module. This is useful when you want to llvmcall intrinsics, because LLVM refuses to parse IR with undeclared identifiers (for example, see this thread).

The idea behind this solution is to allow passing a tuple (decls, ir)as first argument to llvmcall, where currently only an IR string or a pointer to a LLVM function is allowed. The decls IR is then pasted before the function llvmcall generates. In order to avoid duplicate declarations, which LLVM does not like, a global set keeps track of the declarations added to the current module.

Example usage:

function floor(x::Float64)
  llvmcall(
    ("""declare double @llvm.floor.f64(double)""",
     """%2 = call double @llvm.floor.f64(double %0)
        ret double %2"""),
    Float64, (Float64,), x)
end

As mentioned in issue #8308 I also tried other approaches, such as automatically parsing identifiers in the IR, but that turned out pretty fragile.

[maleadt]

I don't get why the OSX buildbot fails -- llvmcallDecls is definitely declared at the top of ccall.cpp...

[tkelman]

If llvmcallDecls is used on every OS, then you should move its declaration before the #if defined(__linux__) || defined(__FreeBSD__) on line 8

[maleadt]

Of course, how silly of me...

[maleadt]

I'm moving this PR over to #8740, sorry for the noise.