[llvm-dwarfdump] Add the --show-sections-sizes option

Add an option to llvm-dwarfdump to calculate the bytes within
the debug sections. Dump this numbers when using --statistics
option as well.

This is an initial patch (e.g. we should support other units,
since we only support 'bytes' now).

Differential Revision: https://reviews.llvm.org/D74205
diff --git a/llvm/docs/CommandGuide/llvm-dwarfdump.rst b/llvm/docs/CommandGuide/llvm-dwarfdump.rst
index b8de084..840c933 100644
--- a/llvm/docs/CommandGuide/llvm-dwarfdump.rst
+++ b/llvm/docs/CommandGuide/llvm-dwarfdump.rst
@@ -105,6 +105,10 @@
             When displaying debug info entries, only show children to a maximum
             depth of <N>.
 
+.. option:: --show-section-sizes
+
+            Show the sizes of all debug sections, expressed in bytes.
+
 .. option:: --statistics
 
             Collect debug info quality metrics and print the results