[llvm] [MC,ELF] Emit warning if a string constant contains newline char. (PR #98060)

[Fangrui Song via llvm-commits]

================
@@ -0,0 +1,69 @@
+// RUN: llvm-mc -filetype=obj -triple x86_64-pc-linux-gnu %s 2>&1 -o /dev/null | FileCheck --strict-whitespace %s --check-prefix=CHECK-WARN
+
+.string "abcd\xFFefg
+12345678"
+
+.ascii "some test ascii
+
+sequence
+with
+newlines\x0A
+"
+
+.asciz "another test string
+
+with
+newline characters
+
+
+"
+
+// CHECK-WARN:   warn-newline-in-escaped-string.s:3:21: warning: unterminated string; newline inserted
----------------
MaskRay wrote:

Instead of string/string/string check/check/check, consider check/string check/string check/string that new tests use more.

You can use the following to test relative line numbers, so that inserting lines in the future would not require test update. We often omit the filename.
```
[[#@LINE+1]]:21: warning: 
```

https://github.com/llvm/llvm-project/pull/98060