[LangRef] Document integer hexadecimal constants

[luke]

Summary

It's possible to write an integer constant in hexadecimal, but you need to
prefix it with u or s. I couldn't find this mentioned anywhere in the LangRef,
so this adds a small note about it.

[arsenm]

Thanks, I’ve known this existed but couldn’t ever remember the syntax. Can you add some more examples?

[luke]

Add some more examples

[luke]

In D158288#4599255, @arsenm wrote:

Thanks, I’ve known this existed but couldn’t ever remember the syntax. Can you add some more examples?

After playing about a bit, I'm actually a bit surprised by how signed hex constants behave. They're sign extended from the number of active bits, so it's not actually possible to represent a positive signed constant. I've traced it back to this commit from 2007: https://github.com/llvm/llvm-project/commit/7957de7949d7f45d0adc81dde38ae0468f1cd09d

I've documented it anyway.

llvm/docs/LangRef.rst

 
 Simple Constants
 ----------------
 
 **Boolean constants**
     The two strings '``true``' and '``false``' are both valid constants
     of the ``i1`` type.
 **Integer constants**
-    Standard integers (such as '4') are constants of the
-    :ref:`integer <t_integer>` type. Negative numbers may be used with
-    integer types.
+    Standard integers (such as '4') are constants of the :ref:`integer
+    <t_integer>` type. They can be either decimal or
+    hexadecimal. Decimal integers can be prefixed with - to represent
+    negative integers, e.g. '``-1234``'. Hexadecimal integers must be
+    prefixed with either u or s to indicate whether they are unsigned
+    or signed respectively. e.g '``u0x8000``' gives 32768, whilst
+    '``s0x8000``' gives -32768.
+
+    Note that hexadecimal integers are sign extended from the number
+    of active bits, i.e. the bit width minus the number of leading
+    zeros. So '``s0x0001``' of type '``i16``' will be -1, not 1.
 **Floating-point constants**
     Floating-point constants use standard decimal notation (e.g.
     123.421), exponential notation (e.g. 1.23421e+2), or a more precise
     hexadecimal notation (see below). The assembler requires the exact
     decimal value of a floating-point constant. For example, the
     assembler accepts 1.25 but rejects 1.3 because 1.3 is a repeating
     decimal in binary. Floating-point constants must have a
     :ref:`floating-point <t_floating>` type.