FileCheck: add page (#15433)

Co-authored-by: Juri Dispan <[email protected]>
Co-authored-by: Sebastiaan Speck <[email protected]>
Co-authored-by: K.B.Dharun Krishna <[email protected]>

pages/common/filecheck.md

@@ -0,0 +1,25 @@
+# FileCheck
+
+> Flexible pattern matching file verifier.
+> It is typically used from LLVM regression tests and forms a part of a `lit` test.
+> More information: <https://llvm.org/docs/CommandGuide/FileCheck.html>.
+
+- Match `input_file` content with pattern file `check_file`:
+
+`FileCheck --input-file={{path/to/input_file}} {{path/to/check_file}}`
+
+- Match input from the `stdin` with pattern file `check_file`:
+
+`echo "{{some_text}}" | FileCheck {{path/to/check_file}}`
+
+- Match with the specified custom check `prefix` (Note: the default prefix is `CHECK`):
+
+`echo "{{some_text}}" | FileCheck --check-prefix={{prefix}} {{path/to/check_file}}`
+
+- Print good directive pattern matches:
+
+`echo "{{some_text}}" | FileCheck -v {{path/to/check_file}}`
+
+- Input `llvm_code.ll` into llvm-as, then pipe the output into FileCheck to match:
+
+`llvm-as {{path/to/llvm_code.ll}} | FileCheck {{path/to/check_file}}`