[profile][test] Improve coverage-linkage.cpp with ld.lld --gc-sections

The __llvm_prf_names section uses SHF_GNU_RETAIN.  However, GNU ld before 2015-10
(https://sourceware.org/bugzilla/show_bug.cgi?id=19161) neither supports it nor
retains __llvm_prf_names according to __start___llvm_prf_names. So --gc-sections
does not work on such old GNU ld.

This is not a problem for gold and sufficiently new lld.

compiler-rt/test/profile/Linux/coverage-linkage.cpp

@@ -1,9 +1,33 @@
 /// Test instrumentation can handle various linkages.
+// REQUIRES: lld-available
 // RUN: %clang_profgen -fcoverage-mapping %s -o %t
 // RUN: env LLVM_PROFILE_FILE=%t.profraw %run %t
+// RUN: llvm-profdata show %t.profraw --all-functions | FileCheck %s
 
-// RUN: %clang_profgen -fcoverage-mapping -ffunction-sections -Wl,--gc-sections %s -o %t
+// RUN: %clang_profgen -fcoverage-mapping -ffunction-sections -fuse-ld=lld -Wl,--gc-sections %s -o %t
 // RUN: env LLVM_PROFILE_FILE=%t.profraw %run %t
+// RUN: llvm-profdata show %t.profraw --all-functions | FileCheck %s
+
+// CHECK:      {{.*}}external{{.*}}:
+// CHECK-NEXT:    Hash:
+// CHECK-NEXT:    Counters: 1
+// CHECK-NEXT:    Function count: 1
+// CHECK:      {{.*}}weak{{.*}}:
+// CHECK-NEXT:    Hash:
+// CHECK-NEXT:    Counters: 1
+// CHECK-NEXT:    Function count: 1
+// CHECK:      main:
+// CHECK-NEXT:    Hash:
+// CHECK-NEXT:    Counters: 1
+// CHECK-NEXT:    Function count: 1
+// CHECK:      {{.*}}internal{{.*}}:
+// CHECK-NEXT:    Hash:
+// CHECK-NEXT:    Counters: 1
+// CHECK-NEXT:    Function count: 1
+// CHECK:      {{.*}}linkonce_odr{{.*}}:
+// CHECK-NEXT:    Hash:
+// CHECK-NEXT:    Counters: 1
+// CHECK-NEXT:    Function count: 1
 
 #include <stdio.h>