[llvm-symbolizer] improve test and fix doc example after recent --print-source-context-lines behaviour change

[bd1976llvm]

Summary

I believe that after https://reviews.llvm.org/D102355 the behaviour of --print-source-context-lines has changed.

Before: --print-source-context-lines=3 prints 4 lines.
After: --print-source-context-lines=3 prints 3 lines.

Adjust the example in the docs for this change and make the testing a little more robust.

A note in passing: This option is a bit confusing. I feel it would be better to follow existing conventions e.g.:

--print-source-context-lines=1 prints 1 line before and after as well as the matching line.
--print-source-after-lines=1 prints 1 line after the matching line as well as the matching line.
--print-source-before-lines=1 prints 1 line before the matching line as well as the matching line.

Maybe -C/-A/-B are available to match grep because the current option is quite long?

[jhenderson]

LGTM.

Maybe -C/-A/-B are available to match grep because the current option is quite long?

We can't adopt new short options from tools other than GNU addr2line, as the command-line syntax of llvm-symbolizer is intended to be compatible (i.e. all addr2line options should be present in llvm-symbolizer). If addr2line were to add one or more of these options with a different meaning, we'd end up with an incompatibility. Also, -C is already used for symbol demangling.

llvm/docs/CommandGuide/llvm-symbolizer.rst

.. code-block:: console
      (inlined by) main at /tmp/test.cpp:15:0
 
 .. option:: --print-source-context-lines <N>
 
   Print ``N`` lines of source context for each symbolized address.
 
   .. code-block:: console
 
-    $ llvm-symbolizer --obj=test.elf 0x400490 --print-source-context-lines=2
+    $ llvm-symbolizer --obj=test.elf 0x400490 --print-source-context-lines=3
     baz()
     /tmp/test.cpp:11:0
     10  :   volatile int k = 42;
     11 >:   return foz() + k;
     12  : }
 
 .. option:: --relativenames
 

llvm/test/tools/llvm-symbolizer/print_context.c

 // RUN: cd %t
 // RUN: llvm-symbolizer -obj=%t/print_context.o 0x0 -print-source-context-lines=5 | FileCheck %s
 
 // Inputs/print_context.o built with plain -g -c from this source file
 // Specifying -Xclang -fdebug-compilation-dir -Xclang . to make the debug info
 // location independent.
 
 // CHECK: inc
-// CHECK: print_context.c:3
-// CHECK: 1  : #include
-// CHECK: 2  :
-// CHECK: 3 >: int inc
-// CHECK: 4  :   return
-// CHECK: 5  : }
+// CHECK-NEXT: print_context.c:3
+// CHECK-NEXT: 1  : #include
+// CHECK-NEXT: 2  :
+// CHECK-NEXT: 3 >: int inc
+// CHECK-NEXT: 4  :   return
+// CHECK-NEXT: 5  : }
+// CHECK-NOT:  6