Fix mutated idents in functions which are inlined more than once

[jwatzman]

The function inlining directly drops the inlined function's body into the place it gets inlined. This is problematic for mutable Idents -- in particular, the renamer could do some very strange things as it tried to rename the same Ident to more than one thing. For example, the included test case was previously renamed to the following:

uniform vec3 v[42];
vec3 N()
{
  return v[4];
}
vec3 N(float v)
{
  return v[4]+v; // <-- Whoops, used "v" for both the global and the local!
}

Therefore all Idents (which aren't function parameters that get substituted away) need to get copied during inlining.

[eldritchconundrum]

I tried to prevent this from happening by detecting it during function inlining candidate analysis, with the rule "Only inline a function if it never refers to a global function or variable by a name that is shadowed by a local variable in scope at the call site", but it looks like I didn't take into account the renaming pass.

[laurentlb]

The side-effect in the renaming pass is a bit annoying (I'd like to move the new name from Ident to VarDecl). Making a copy makes sense to me.

Thanks!