Go back to using separate grammar productions.

This changes template-args to expression in the grammar as well, which
was the original suggestion that I misunderstood.

Author: AaronBallman

abi.html

@@ -5014,8 +5014,10 @@ <h5><a href="#mangling-builtin">5.1.5.2 Builtin types</a></h5>
                  ::= Df # IEEE 754r decimal floating point (32 bits)
                  ::= Dh # IEEE 754r half-precision floating point (16 bits)
                  ::= DF &lt;<a href="#mangle.number">number</a>&gt; _ # ISO/IEC TS 18661 binary floating point type _FloatN (N bits)
-                 ::= DB &lt;<a href="#mangle.number">number</a>&gt; | &lt;<a href="#mangle.template-args">template-args</a>&gt; _  # C23 signed _BitInt(N)
-                 ::= DU &lt;<a href="#mangle.number">number</a>&gt; | &lt;<a href="#mangle.template-args">template-args</a>&gt; _  # C23 unsigned _BitInt(N)
+                 ::= DB &lt;<a href="#mangle.number">number</a>&gt; _        # C23 signed _BitInt(N)
+                 ::= DB &lt;<a href="#mangle.expression">expression</a>&gt; _ # C23 signed _BitInt(N)
+                 ::= DU &lt;<a href="#mangle.number">number</a>&gt; _        # C23 unsigned _BitInt(N)
+                 ::= DU &lt;<a href="#mangle.expression">expression</a>&gt; _ # C23 unsigned _BitInt(N)
                  ::= Di # char32_t
                  ::= Ds # char16_t
                  ::= Du # char8_t