[Profile] Rewrite Test to work with Internal Shell

There was one test that used subshells to read a file. Replace those
subshells with the readfile substitution.

Reviewers: fmayer, mingmingl-llvm

Reviewed By: mingmingl-llvm, fmayer

Pull Request: https://github.com/llvm/llvm-project/pull/165145

compiler-rt/test/profile/instrprof-hostname.c

@@ -1,7 +1,7 @@
 // RUN: %clang_profgen -o %t -O3 %s
 // RUN: env LLVM_PROFILE_FILE=%h.%t-%h.profraw_%h %run %t
-// RUN: %run uname -n > %t.n
-// RUN: llvm-profdata merge -o %t.profdata `cat %t.n`.%t-`cat %t.n`.profraw_`cat %t.n`
+// RUN: %run uname -n | tr -d '\n' > %t.n
+// RUN: llvm-profdata merge -o %t.profdata %{readfile:%t.n}.%t-%{readfile:%t.n}.profraw_%{readfile:%t.n}
 // RUN: %clang_profuse=%t.profdata -o - -S -emit-llvm %s | FileCheck %s
 // REQUIRES: shell