[Tests] Add system-cygwin feature, and use it.

Several Clang tests were failing on Cygwin, and were already marked as
requiring !system-windows, unsupported on system-windows, or xfail on
system-windows.  Add system-cygwin to lit's llvm.config, and use it in
such tests in addition to system-windows.

Author: jeremyd2019

clang/test/CodeGen/2007-06-18-SextAttrAggregate.c

@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -no-enable-noundef-analysis %s -o - -emit-llvm | FileCheck %s
-// XFAIL: target={{(aarch64|arm64).*}}, target=x86_64-pc-windows-msvc, target=x86_64-{{(pc|w64)}}-windows-gnu
+// XFAIL: target={{(aarch64|arm64).*}}, target=x86_64-pc-windows-msvc, target=x86_64-{{(pc|w64)}}-windows-gnu, target=x86_64-pc-windows-cygnus
 
 // PR1513
 

clang/test/CodeGen/debug-info-version.c

@@ -1,4 +1,4 @@
-// REQUIRES: !system-windows
+// REQUIRES: !system-windows, !system-cygwin
 // RUN: %clang -g -S -emit-llvm -o - %s | FileCheck %s
 // RUN: %clang -S -emit-llvm -o - %s | FileCheck %s --check-prefix=NO_DEBUG
 int main (void) {

clang/test/CodeGenCXX/modules-vtable.cppm

@@ -1,4 +1,4 @@
-// REQUIRES: !system-windows
+// REQUIRES: !system-windows, !system-cygwin
 
 // RUN: rm -rf %t
 // RUN: split-file %s %t

clang/test/CodeGenCXX/pr70585.cppm

@@ -1,4 +1,4 @@
-// REQUIRES: !system-windows
+// REQUIRES: !system-windows, !system-cygwin
 
 // RUN: rm -rf %t
 // RUN: split-file %s %t

clang/test/Driver/cuda-detect-path.cu

@@ -1,5 +1,5 @@
 // This tests uses the PATH environment variable.
-// REQUIRES: !system-windows
+// REQUIRES: !system-windows, !system-cygwin
 
 // RUN: env PATH=%S/Inputs/CUDA/usr/local/cuda/bin \
 // RUN:    %clang -v --target=i386-unknown-linux --sysroot=%S/no-cuda-there \

clang/test/Driver/hipspv-toolchain.hip

@@ -1,4 +1,4 @@
-// UNSUPPORTED: system-windows
+// UNSUPPORTED: system-windows, system-cygwin
 
 // RUN: %clang -### -target x86_64-linux-gnu --offload=spirv64 \
 // RUN:   --no-offload-new-driver --hip-path=%S/Inputs/hipspv -nohipwrapperinc %s \

clang/test/Driver/ld-path.c

@@ -1,5 +1,5 @@
 /// This tests uses the PATH environment variable.
-// UNSUPPORTED: system-windows
+// UNSUPPORTED: system-windows, system-cygwin
 
 // RUN: cd %S
 

clang/test/Driver/program-path-priority.c

@@ -1,5 +1,5 @@
 /// Don't create symlinks on Windows
-// UNSUPPORTED: system-windows
+// UNSUPPORTED: system-windows, system-cygwin
 
 /// Check the priority used when searching for tools
 /// Names and locations are usually in this order:

clang/test/Driver/spirv-toolchain.cl

@@ -92,7 +92,7 @@
 // RUN: mkdir -p %t/versioned
 // RUN: touch %t/versioned/spirv-as-%llvm-version-major \
 // RUN:   && chmod +x %t/versioned/spirv-as-%llvm-version-major
-// RUN: %if !system-windows %{ env "PATH=%t/versioned" %clang -### --target=spirv64 -x cl -c --save-temps %s 2>&1 \
+// RUN: %if !system-windows && !system-cygwin %{ env "PATH=%t/versioned" %clang -### --target=spirv64 -x cl -c --save-temps %s 2>&1 \
 // RUN:   | FileCheck -DVERSION=%llvm-version-major --check-prefix=VERSIONED %s %}
 
 // VERSIONED: {{.*}}spirv-as-[[VERSION]]

clang/test/Interpreter/simple-exception.cpp

@@ -1,7 +1,7 @@
 // clang-format off
 // UNSUPPORTED: system-aix
 // XFAIL for arm, or running on Windows.
-// XFAIL: target=arm-{{.*}}, target=armv{{.*}}, system-windows
+// XFAIL: target=arm-{{.*}}, target=armv{{.*}}, system-windows, system-cygwin
 // RUN: cat %s | clang-repl | FileCheck %s
 
 // Incompatible with msan. It passes with -O3 but fail -Oz. Interpreter