[NFC] Adding missing flang tests for LTO bugfix

Pull request #78749 introduced a fix for passing vector library flags
from clang frontend to opt when LTO is used.

The fix also covered flang, as it's handled in CommonArgs.cpp, but no
testing was added for flang.

Author: paschalis-mpeis

flang/test/Driver/fveclib.f90

@@ -30,3 +30,20 @@
 
 ! TODO: if we add support for -nostdlib or -nodefaultlibs we need to test that
 ! these prevent "-framework Accelerate" being added on Darwin
+
+! Verify that the correct vector library is passed to LTO flags.
+
+! RUN: %flang -### --target=x86_64-unknown-linux-gnu -fveclib=LIBMVEC -flto %s 2>&1 | FileCheck -check-prefix CHECK-LTO-LIBMVEC %s
+! CHECK-LTO-LIBMVEC: "-plugin-opt=-vector-library=LIBMVEC-X86"
+
+! RUN: %flang -### --target=powerpc64-unknown-linux-gnu -fveclib=MASSV -flto %s 2>&1 | FileCheck -check-prefix CHECK-LTO-MASSV %s
+! CHECK-LTO-MASSV: "-plugin-opt=-vector-library=MASSV"
+
+! RUN: %flang -### --target=x86_64-unknown-linux-gnu -fveclib=SVML -flto %s 2>&1 | FileCheck -check-prefix CHECK-LTO-SVML %s
+! CHECK-LTO-SVML: "-plugin-opt=-vector-library=SVML"
+
+! RUN: %flang -### --target=aarch64-linux-gnu -fveclib=SLEEF -flto %s 2>&1 | FileCheck -check-prefix CHECK-LTO-SLEEF %s
+! CHECK-LTO-SLEEF: "-plugin-opt=-vector-library=sleefgnuabi"
+
+! RUN: %flang -### --target=aarch64-linux-gnu -fveclib=ArmPL -flto %s 2>&1 | FileCheck -check-prefix CHECK-LTO-ARMPL %s
+! CHECK-LTO-ARMPL: "-plugin-opt=-vector-library=ArmPL"