Fix parsing of nested generic app. (shader-slang#6259)

Author: csyonghe

tests/front-end/generic-disambiguate-module.slang

@@ -0,0 +1,2 @@
+//TEST_IGNORE_FILE:
+struct Foo<T> {}

tests/front-end/generic-disambiguate.slang

@@ -0,0 +1,10 @@
+//TEST:SIMPLE(filecheck=CHECK): -target spirv
+import "generic-disambiguate-module";
+
+// CHECK: OpEntryPoint
+
+[numthreads(1,1,1)]
+void main()
+{
+    var x : Foo<Foo<Foo<Foo<float>>>>; //   error 20002: syntax error.
+}