Fix codegen regression in skip_whitespace() from #5490

Benchmarking found the get()/get_ignoring_pending_unget() split in
skip_whitespace() made long whitespace runs (e.g. indentation in
pretty-printed JSON) 1.75x-3.2x SLOWER instead of faster, reproducible
with both Apple Clang and GCC.

Root cause: rewriting the loop from a plain do-while into an initial
get() followed by a while-loop defeated the compiler's ability to keep
the input adapter's read/end pointers in registers across iterations;
both compilers instead reloaded them from memory on every character.
The function split itself was not the problem (it still fully
inlines); the loop's control-flow shape was.

The fix keeps the same two-function structure but restores a
do-while shape (guarded by an if for the "first char not whitespace"
case), which lets both compilers hoist the pointers back into
registers, matching or beating pre-#5490 performance.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

include/nlohmann/detail/input/lexer.hpp

@@ -1705,9 +1705,20 @@ scan_number_done:
         // nothing below calls unget()
         get();
 
-        while (current == ' ' || current == '\t' || current == '\n' || current == '\r')
+        // this is written as an if-guarded do-while (rather than a plain
+        // while loop) because that shape is what lets both GCC and Clang
+        // keep the input adapter's read pointer in a register across
+        // iterations; the equivalent while-loop measurably defeated that
+        // optimization in testing, turning long whitespace runs (e.g. the
+        // indentation of pretty-printed JSON) from a register-only loop
+        // into one that reloads the pointer from memory every character
+        if (current == ' ' || current == '\t' || current == '\n' || current == '\r')
         {
-            get_ignoring_pending_unget();
+            do
+            {
+                get_ignoring_pending_unget();
+            }
+            while (current == ' ' || current == '\t' || current == '\n' || current == '\r');
         }
     }