parser: Add testcases for multiline strings

Regression checks for Rust-GCC#1399

gcc/testsuite/ChangeLog:

	* rust/compile/multiline-string.rs: New test.
	* rust/execute/torture/multiline-string.rs: New test.

gcc/testsuite/rust/compile/multiline-string.rs

@@ -0,0 +1,14 @@
+fn main() {
+    let _a = "gcc
+    
+    rs";
+
+    let _b = "rust
+    
+    c
+    gcc
+    
+    
+    
+    rs";
+}

gcc/testsuite/rust/execute/torture/multiline-string.rs

@@ -0,0 +1,13 @@
+// { dg-output "gcc\n\nrs" }
+
+extern "C" {
+    fn printf(fmt: *const i8, ...);
+}
+
+fn main() {
+    let a = "gcc
+
+rs\0";
+
+    unsafe { printf("%s\n\0" as *const str as *const i8, a as *const str as *const i8); }
+}