Fix test of -print-multi-flags-experimental in case of multilib custom flags (#123577)

The test was failing in the case where a `multilib.yaml` file was
present in the installation. This is because the presence of a multilib
YAML file leads to the diagnosing of validity of the multilib custom
flags.

This patch fixes the test by creating a new YAML file with multilib
custom flags to be used by the test.

Author: vhscampos

clang/test/Driver/Inputs/multilib/multilib-custom-flags.yaml

@@ -0,0 +1,13 @@
+MultilibVersion: 1.0
+
+Variants:
+
+Mappings:
+
+Flags:
+- Name: flag
+  Values:
+  - Name: foo
+  - Name: bar
+  Default: foo
+...

clang/test/Driver/print-multi-selection-flags.c

@@ -91,8 +91,8 @@
 // CHECK-RV32E-ORDER: -mabi=ilp32e
 // CHECK-RV32E-ORDER: -march=rv32e{{[0-9]+p[0-9]+}}_c{{[0-9]+p[0-9]+}}_zicsr{{[0-9]+p[0-9]+}}
 
-// RUN: %clang -print-multi-flags-experimental --target=armv8m.main-none-eabi -fmultilib-flag=foo -fmultilib-flag=bar | FileCheck --check-prefixes=CHECK-MULTILIB-CUSTOM-FLAG,CHECK-ARM-MULTILIB-CUSTOM-FLAG %s
-// RUN: %clang -print-multi-flags-experimental --target=aarch64-none-eabi     -fmultilib-flag=foo -fmultilib-flag=bar | FileCheck --check-prefixes=CHECK-MULTILIB-CUSTOM-FLAG,CHECK-AARCH64-MULTILIB-CUSTOM-FLAG %s
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/multilib-custom-flags.yaml -print-multi-flags-experimental --target=armv8m.main-none-eabi -fmultilib-flag=foo -fmultilib-flag=bar | FileCheck --check-prefixes=CHECK-MULTILIB-CUSTOM-FLAG,CHECK-ARM-MULTILIB-CUSTOM-FLAG %s
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/multilib-custom-flags.yaml -print-multi-flags-experimental --target=aarch64-none-eabi     -fmultilib-flag=foo -fmultilib-flag=bar | FileCheck --check-prefixes=CHECK-MULTILIB-CUSTOM-FLAG,CHECK-AARCH64-MULTILIB-CUSTOM-FLAG %s
 // CHECK-ARM-MULTILIB-CUSTOM-FLAG:     --target=thumbv8m.main-unknown-none-eabi
 // CHECK-AARCH64-MULTILIB-CUSTOM-FLAG: --target=aarch64-unknown-none-eabi
 // CHECK-MULTILIB-CUSTOM-FLAG-DAG:     -fmultilib-flag=foo