Update the codegen/mainsubprogram tests to min-llvm 4.0

The necessary changes were only in upstream LLVM in 4.0, but they were
for a while backported to Rust LLVM.  Now that Rust LLVM is also 4.0, we
can make the test conditional here more accurate.

src/test/codegen/mainsubprogram.rs

@@ -8,14 +8,13 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// The minimum LLVM version is set to 3.8, but really this test
-// depends on a patch that is was committed to upstream LLVM before
-// 4.0; and also backported to the Rust LLVM fork.
+// This test depends on a patch that was committed to upstream LLVM
+// before 4.0, formerly backported to the Rust LLVM fork.
 
 // ignore-tidy-linelength
 // ignore-windows
 // ignore-macos
-// min-llvm-version 3.8
+// min-llvm-version 4.0
 
 // compile-flags: -g -C no-prepopulate-passes
 

src/test/codegen/mainsubprogramstart.rs

@@ -8,14 +8,13 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// The minimum LLVM version is set to 3.8, but really this test
-// depends on a patch that is was committed to upstream LLVM before
-// 4.0; and also backported to the Rust LLVM fork.
+// This test depends on a patch that was committed to upstream LLVM
+// before 4.0, formerly backported to the Rust LLVM fork.
 
 // ignore-tidy-linelength
 // ignore-windows
 // ignore-macos
-// min-llvm-version 3.8
+// min-llvm-version 4.0
 
 // compile-flags: -g -C no-prepopulate-passes