Upgrades the coverage map to Version 4

Changes the coverage map injected into binaries compiled with
`-Zinstrument-coverage` to LLVM Coverage Mapping Format, Version 4 (from
Version 3). Note, binaries compiled with this version will require LLVM
tools from at least LLVM Version 11.

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -220,12 +220,24 @@ pub fn set_linkage(llglobal: &Value, linkage: Linkage) {
     }
 }
 
+pub fn set_visibility(llglobal: &Value, visibility: Visibility) {
+    unsafe {
+        LLVMRustSetVisibility(llglobal, visibility);
+    }
+}
+
 pub fn set_alignment(llglobal: &Value, bytes: usize) {
     unsafe {
         ffi::LLVMSetAlignment(llglobal, bytes as c_uint);
     }
 }
 
+pub fn set_comdat(llmod: &Module, llglobal: &Value, name: &str) {
+    unsafe {
+        LLVMRustSetComdat(llmod, llglobal, name.as_ptr().cast(), name.len());
+    }
+}
+
 /// Safe wrapper around `LLVMGetParam`, because segfaults are no fun.
 pub fn get_param(llfn: &Value, index: c_uint) -> &Value {
     unsafe {