Replace LLVMConstInBoundsGEP with LLVMConstInBoundsGEP2*

A custom reimplementation of LLVMConstInBoundsGEP2 is used, since the
LLVM contains a declaration of LLVMConstInBoundsGEP2 but not the
implementation.

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1011,7 +1011,8 @@ extern "C" {
     pub fn LLVMConstVector(ScalarConstantVals: *const &Value, Size: c_uint) -> &Value;
 
     // Constant expressions
-    pub fn LLVMConstInBoundsGEP(
+    pub fn LLVMRustConstInBoundsGEP2(
+        ty: &'a Type,
         ConstantVal: &'a Value,
         ConstantIndices: *const &'a Value,
         NumIndices: c_uint,