[Driver,test] Remove -v from mkdir & ln -s from a test since it fails on AIX (#72924)

The PR #70817 introduced a
small bug, the tests failed on ppc64-aix with:

```
RUN: at line 186: mkdir -pv /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/tools/clang/test/Driver/Output/darwin-header-search-libcxx.cpp.tmp/install/bin
+ mkdir -pv /home/powerllvm/powerllvm_env/aix-ppc64/clang-ppc64-aix/build/tools/clang/test/Driver/Output/darwin-header-search-libcxx.cpp.tmp/install/bin
mkdir: illegal option -- v
Usage: mkdir [-p] [-e] [-m mode] Directory ... 
```

This PR removes the verbose flag from both `mkdir` and `ln -s`.

Author: ilg-ul

clang/test/Driver/darwin-header-search-libcxx.cpp

@@ -183,15 +183,15 @@
 // local folder hierarchy that meets this requirement.
 // Note: this might not work with weird RPATH configurations.
 // RUN: rm -rf %t/install
-// RUN: mkdir -pv %t/install/bin
+// RUN: mkdir -p %t/install/bin
 // RUN: cp %clang %t/install/bin/clang
-// RUN: mkdir -pv %t/install/include/c++/v1
+// RUN: mkdir -p %t/install/include/c++/v1
 
 // Headers in (1) and in (2) -> (1) is preferred over (2)
 // RUN: rm -rf %t/symlinked1
-// RUN: mkdir -pv %t/symlinked1/bin
-// RUN: ln -svf %t/install/bin/clang %t/symlinked1/bin/clang
-// RUN: mkdir -pv %t/symlinked1/include/c++/v1
+// RUN: mkdir -p %t/symlinked1/bin
+// RUN: ln -sf %t/install/bin/clang %t/symlinked1/bin/clang
+// RUN: mkdir -p %t/symlinked1/include/c++/v1
 
 // RUN: %t/symlinked1/bin/clang -### %s -fsyntax-only 2>&1 \
 // RUN:     --target=x86_64-apple-darwin \
@@ -207,8 +207,8 @@
 
 // Headers in (2) and in (3) -> (2) is preferred over (3)
 // RUN: rm -rf %t/symlinked2
-// RUN: mkdir -pv %t/symlinked2/bin
-// RUN: ln -svf %t/install/bin/clang %t/symlinked2/bin/clang
+// RUN: mkdir -p %t/symlinked2/bin
+// RUN: ln -sf %t/install/bin/clang %t/symlinked2/bin/clang
 
 // RUN: %t/symlinked2/bin/clang -### %s -fsyntax-only 2>&1 \
 // RUN:     --target=x86_64-apple-darwin \