Add support for relative line number expressions

For example:
```
// CHECK: file.c:[[# @line + 3]]:8: error
```

Author: obycode

docs/05-check-commands.rst

@@ -175,3 +175,13 @@ If the empty line is removed, the test will fail:
     ^
     $ echo $?
     1
+
+Line number expression
+======================
+
+It is often useful to check for a specific line number in your regular
+expression, relative to its location in the file. Hard-coding that number can
+make the test fragile -- rearranging, adding, or deleting lines requires
+changing the expression. To solve this, FileCheck supports a variable for the
+current line number, `[[# @LINE ]]`, as well as simple offsets from this
+variable, e.g. `[[# @LINE + 4 ]]` or `[[# @LINE - 2 ]]`.

filecheck/FileCheck.py

@@ -317,6 +317,18 @@ def exit_handler(code):
                         if check_expression[-1] != "$":
                             check_expression = check_expression + "$"
 
+                # Replace line number expressions, e.g. `[[# @LINE + 3 ]]`
+                line_var_match = re.search(
+                    r"\[\[# +@LINE *([+-])? *([0-9]+)? *\]\]", check_expression)
+                while line_var_match is not None:
+                    offset = int(line_var_match.group(2) or 0)
+                    if line_var_match.group(1) == "-":
+                        offset = -offset
+                    check_expression = re.sub(
+                        r"\[\[# +@LINE *([+-])? *([0-9]+)? *\]\]", str(line_idx + offset + 1), check_expression)
+                    line_var_match = re.search(
+                        r"\[\[# +@LINE *([+-])? *([0-9]+)? *\]\]", check_expression)
+
                 check = Check(check_type=check_type,
                               match_type=match_type,
                               check_keyword=check_keyword,

tests/integration/tests/examples/lit-and-filecheck/03-line.c

@@ -0,0 +1,10 @@
+/**
+; RUN: gcc "%s" -o %S/line && %S/line | filecheck %s
+ */
+
+#include <stdio.h>
+int main() {
+  // CHECK: Hello from line [[# @LINE + 1 ]]
+  printf("Hello from line %d\n", __LINE__);
+  return 0;
+}