Rename variadic to c_variadic

Function signatures with the `variadic` member set are actually
C-variadic functions. Make this a little more explicit by renaming the
`variadic` boolean value, `c_variadic`.

src/test/ui/c-variadic/variadic-ffi-1.stderr

@@ -1,8 +1,8 @@
-error[E0045]: variadic function must have C or cdecl calling convention
+error[E0045]: C-variadic function must have C or cdecl calling convention
   --> $DIR/variadic-ffi-1.rs:5:5
    |
 LL |     fn printf(_: *const u8, ...); //~ ERROR: variadic function must have C or cdecl calling
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ variadics require C or cdecl calling convention
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadics require C or cdecl calling convention
 
 error: aborting due to previous error
 

src/test/ui/c-variadic/variadic-ffi-2.stderr

@@ -1,8 +1,8 @@
-error[E0045]: variadic function must have C or cdecl calling convention
+error[E0045]: C-variadic function must have C or cdecl calling convention
   --> $DIR/variadic-ffi-2.rs:3:11
    |
 LL | fn baz(f: extern "stdcall" fn(usize, ...)) {
-   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ variadics require C or cdecl calling convention
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadics require C or cdecl calling convention
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0045.stderr

@@ -1,8 +1,8 @@
-error[E0045]: variadic function must have C or cdecl calling convention
+error[E0045]: C-variadic function must have C or cdecl calling convention
   --> $DIR/E0045.rs:1:17
    |
 LL | extern "Rust" { fn foo(x: u8, ...); }   //~ ERROR E0045
-   |                 ^^^^^^^^^^^^^^^^^^^ variadics require C or cdecl calling convention
+   |                 ^^^^^^^^^^^^^^^^^^^ C-variadics require C or cdecl calling convention
 
 error: aborting due to previous error
 

src/test/ui/invalid/invalid-variadic-function.rs

@@ -1,3 +1,3 @@
 extern "C" fn foo(x: u8, ...);
-//~^ ERROR only foreign functions are allowed to be variadic
+//~^ ERROR only foreign functions are allowed to be C-variadic
 //~| ERROR expected one of `->`, `where`, or `{`, found `;`

src/test/ui/invalid/invalid-variadic-function.stderr

@@ -1,4 +1,4 @@
-error: only foreign functions are allowed to be variadic
+error: only foreign functions are allowed to be C-variadic
   --> $DIR/invalid-variadic-function.rs:1:26
    |
 LL | extern "C" fn foo(x: u8, ...);

src/test/ui/parser/variadic-ffi-3.rs

@@ -1,5 +1,5 @@
 fn foo(x: isize, ...) {
-    //~^ ERROR: only foreign functions are allowed to be variadic
+    //~^ ERROR: only foreign functions are allowed to be C-variadic
 }
 
 fn main() {}

src/test/ui/parser/variadic-ffi-3.stderr

@@ -1,4 +1,4 @@
-error: only foreign functions are allowed to be variadic
+error: only foreign functions are allowed to be C-variadic
   --> $DIR/variadic-ffi-3.rs:1:18
    |
 LL | fn foo(x: isize, ...) {

src/test/ui/parser/variadic-ffi-4.rs

@@ -1,5 +1,5 @@
 extern "C" fn foo(x: isize, ...) {
-    //~^ ERROR: only foreign functions are allowed to be variadic
+    //~^ ERROR: only foreign functions are allowed to be C-variadic
 }
 
 fn main() {}

src/test/ui/parser/variadic-ffi-4.stderr

@@ -1,4 +1,4 @@
-error: only foreign functions are allowed to be variadic
+error: only foreign functions are allowed to be C-variadic
   --> $DIR/variadic-ffi-4.rs:1:29
    |
 LL | extern "C" fn foo(x: isize, ...) {