[NFC][clang] Quote literal builtin attribute markers in Builtins.def docs (#160080)

Clarify the documentation in Builtins.def by quoting literal attribute
markers (e.g. 'n', 'r', 'U', 'N') to distinguish them from placeholders
such as N in C<N,...>. This avoids confusion and makes the attribute
docs clearer.

Author: bozicrHT

clang/include/clang/Basic/Builtins.def

@@ -66,7 +66,8 @@
 
 // The third value provided to the macro specifies information about attributes
 // of the function.  These must be kept in sync with the predicates in the
-// Builtin::Context class.  Currently we have:
+// Builtin::Context class.  Note: In the descriptions below, {num} is a
+// placeholder for an integer.  Currently we have:
 //  n -> nothrow
 //  r -> noreturn
 //  U -> pure
@@ -82,23 +83,23 @@
 //  h -> this function requires a specific header or an explicit declaration.
 //  i -> this is a runtime library implemented function without the
 //       '__builtin_' prefix. It will be implemented in compiler-rt or libgcc.
-//  p:N: -> this is a printf-like function whose Nth argument is the format
-//          string.
-//  P:N: -> similar to the p:N: attribute, but the function is like vprintf
-//          in that it accepts its arguments as a va_list rather than
-//          through an ellipsis
-//  s:N: -> this is a scanf-like function whose Nth argument is the format
-//          string.
-//  S:N: -> similar to the s:N: attribute, but the function is like vscanf
-//          in that it accepts its arguments as a va_list rather than
-//          through an ellipsis
+//  p:{num}: -> this is a printf-like function whose {num}th argument is the
+//              format string.
+//  P:{num}: -> similar to the p:{num}: attribute, but the function is like
+//              vprintf in that it accepts its arguments as a va_list rather than
+//              through an ellipsis
+//  s:{num}: -> this is a scanf-like function whose {num}th argument is the
+//              format string.
+//  S:{num}: -> similar to the s:{num}: attribute, but the function is like
+//              vscanf in that it accepts its arguments as a va_list rather than
+//              through an ellipsis
 //  e -> const, but only when -fno-math-errno and FP exceptions are ignored
 //  g -> const when FP exceptions are ignored
 //  j -> returns_twice (like setjmp)
 //  u -> arguments are not evaluated for their side-effects
-//  V:N: -> requires vectors of at least N bits to be legal
-//  C<N,M_0,...,M_k> -> callback behavior: argument N is called with argument
-//                      M_0, ..., M_k as payload
+//  V:{num}: -> requires vectors of at least {num} bits to be legal
+//  C<{num},M_0,...,M_k> -> callback behavior: argument {num} is called with
+//                          argument M_0, ..., M_k as payload
 //  z -> this is a function in (possibly-versioned) namespace std
 //  E -> this function can be constant evaluated by Clang frontend
 //  G -> this is a C++20 consteval function