Test for link time constants

This test exercises link time constants used by modules that are precompiled to SPIR-V.

Work on shader-slang#6524

Author: cheneym2

tests/library/export-library-global.slang

@@ -0,0 +1,7 @@
+// TEST_IGNORE_FILE
+
+// export-library-global.slang
+
+export static int myGlobalVar = 42;
+
+

tests/library/precompiled-spirv-global.slang

@@ -0,0 +1,12 @@
+//TEST:COMPILE: tests/library/precompiled-spirv-global.slang -o tests/library/precompiled-spirv-global.slang-module -target spirv -embed-downstream-ir
+
+//import "export-library-global.slang";  //This would make the test pass
+
+extern int myGlobalVar; //This makes the test fail
+
+layout(location = 0) out float4 fragColor;
+
+[shader("fragment")]
+void main() {
+    fragColor = float4(myGlobalVar);
+}