Don't try to delete nonexistent preprocessor output

Clang can remove the output file on its own if it fails, so only delete the file if it hasn't already.

Adds (integration) tests.

Fixes #1235.

src/parsers/vct/parsers/parser/ColCPPParser.scala

@@ -94,6 +94,6 @@ case class ColCPPParser(
       val result = ColIPPParser(debugOptions, blameProvider, Some(baseOrigin))
         .parse[G](ireadable)
       result
-    } finally { Files.delete(interpreted) }
+    } finally { Files.deleteIfExists(interpreted) }
   }
 }

src/parsers/vct/parsers/parser/ColCParser.scala

@@ -94,6 +94,6 @@ case class ColCParser(
       val result = ColIParser(debugOptions, blameProvider, Some(baseOrigin))
         .parse[G](ireadable)
       result
-    } finally { Files.delete(interpreted) }
+    } finally { Files.deleteIfExists(interpreted) }
   }
 }

test/main/vct/test/integration/examples/CPPSpec.scala

@@ -23,4 +23,9 @@ class CPPSpec extends VercorsSpec {
   vercors should verify using silicon example "concepts/cpp/methods/Permissions.cpp"
   vercors should verify using silicon example "concepts/cpp/methods/Predicates.cpp"
   vercors should verify using silicon example "concepts/cpp/methods/PureGhostMethod.cpp"
+
+  vercors should error withCode "preprocessorError" in "Source file with preprocessor error" cpp
+  """
+  #define foo(
+  """
 }

test/main/vct/test/integration/examples/CSpec.scala

@@ -346,6 +346,11 @@ class CSpec extends VercorsSpec {
     }
     """
 
+    vercors should error withCode "preprocessorError" in "Source file with preprocessor error" c
+    """
+    #define foo(
+    """
+
     vercors should verify using silicon in "Parallel omp loop with declarations inside" c
     """
     #include <omp.h>

test/main/vct/test/integration/helper/VercorsSpec.scala

@@ -212,26 +212,17 @@ abstract class VercorsSpec extends AnyFlatSpec {
   }
 
   class DescPhrase(val verdict: Verdict, val backends: Seq[Backend], val desc: String, val flags: Seq[String]) {
-    def pvl(data: String)(implicit pos: source.Position): Unit = {
-      val inputs = Seq(LiteralReadable("test.pvl", data))
+    private def literal(suffix: String, data: String)(implicit pos: source.Position): Unit = {
+      val inputs = Seq(LiteralReadable(s"test.$suffix", data))
       for(backend <- backends) {
         registerTest(verdict, desc, Seq(new Tag("literalCase")), backend, inputs, flags)
       }
     }
 
-    def java(data: String)(implicit pos: source.Position): Unit = {
-      val inputs = Seq(LiteralReadable("test.java", data))
-      for(backend <- backends) {
-        registerTest(verdict, desc, Seq(new Tag("literalCase")), backend, inputs, flags)
-      }
-    }
-
-    def c(data: String)(implicit pos: source.Position): Unit = {
-      val inputs = Seq(LiteralReadable("test.c", data))
-      for(backend <- backends) {
-        registerTest(verdict, desc, Seq(new Tag("literalCase")), backend, inputs, flags)
-      }
-    }
+    def pvl(data: String)(implicit pos: source.Position): Unit = literal("pvl", data)
+    def java(data: String)(implicit pos: source.Position): Unit = literal("java", data)
+    def c(data: String)(implicit pos: source.Position): Unit = literal("c", data)
+    def cpp(data: String)(implicit pos: source.Position): Unit = literal("cpp", data)
   }
 
   val vercors: VercorsWord = new VercorsWord