LangRef: add a note about the mangling-suppressing \01 prefix

[hans]

Summary

Someone asked about this on IRC the other day, and I couldn't find the magic prefix documented anywhere.

[rnk]

lgtm

We should make the windows name mangler treat '?' as special so we don't need to add \01 to all C++ names. It's been slowly driving me insane.

[hans]

Closed by commit rL214329 (authored by @hans).

llvm/trunk/docs/LangRef.rst

 
 #. Named values are represented as a string of characters with their
    prefix. For example, ``%foo``, ``@DivisionByZero``,
    ``%a.really.long.identifier``. The actual regular expression used is
    '``[%@][a-zA-Z$._][a-zA-Z$._0-9]*``'. Identifiers which require other
    characters in their names can be surrounded with quotes. Special
    characters may be escaped using ``"\xx"`` where ``xx`` is the ASCII
    code for the character in hexadecimal. In this way, any character can
-   be used in a name value, even quotes themselves.
+   be used in a name value, even quotes themselves. The ``"\01"`` prefix
+   can be used on global variables to suppress mangling.
 #. Unnamed values are represented as an unsigned numeric value with
    their prefix. For example, ``%12``, ``@2``, ``%44``.
 #. Constants, which are described in the section  Constants_ below.
 
 LLVM requires that values start with a prefix for two reasons: Compilers
 don't need to worry about name clashes with reserved words, and the set
 of reserved words may be expanded in the future without penalty.
 Additionally, unnamed identifiers allow a compiler to quickly come up