[libc] fix stdbit include test when not all entrypoints are available

libc/test/include/stdbit_test.cpp

@@ -8,26 +8,38 @@
 
 #include "test/UnitTest/Test.h"
 
-#include <stdbit.h>
-
 /*
  * The intent of this test is validate that:
- * 1. We provide the definition of the various type generic macros of stdbit.h.
+ * 1. We provide the definition of the various type generic macros of stdbit.h
+ * (the macros are transitively included from stdbit-macros.h by stdbit.h).
  * 2. It dispatches to the correct underlying function.
  * Because unit tests build without public packaging, the object files produced
  * do not contain non-namespaced symbols.
  */
 
-unsigned char stdc_leading_zeros_uc(unsigned char) { return 0xAA; }
-unsigned short stdc_leading_zeros_us(unsigned short) { return 0xAB; }
-unsigned stdc_leading_zeros_ui(unsigned) { return 0xAC; }
-unsigned long stdc_leading_zeros_ul(unsigned long) { return 0xAD; }
-unsigned long long stdc_leading_zeros_ull(unsigned long long) { return 0xAF; }
-unsigned char stdc_leading_ones_uc(unsigned char) { return 0xBA; }
-unsigned short stdc_leading_ones_us(unsigned short) { return 0xBB; }
-unsigned stdc_leading_ones_ui(unsigned) { return 0xBC; }
-unsigned long stdc_leading_ones_ul(unsigned long) { return 0xBD; }
-unsigned long long stdc_leading_ones_ull(unsigned long long) { return 0xBF; }
+/*
+ * Declare these BEFORE including stdbit-macros.h so that this test may still be
+ * run even if a given target doesn't yet have these individual entrypoints
+ * enabled.
+ */
+extern "C" {
+unsigned char stdc_leading_zeros_uc(unsigned char) noexcept { return 0xAA; }
+unsigned short stdc_leading_zeros_us(unsigned short) noexcept { return 0xAB; }
+unsigned stdc_leading_zeros_ui(unsigned) noexcept { return 0xAC; }
+unsigned long stdc_leading_zeros_ul(unsigned long) noexcept { return 0xAD; }
+unsigned long long stdc_leading_zeros_ull(unsigned long long) noexcept {
+  return 0xAF;
+}
+unsigned char stdc_leading_ones_uc(unsigned char) noexcept { return 0xBA; }
+unsigned short stdc_leading_ones_us(unsigned short) noexcept { return 0xBB; }
+unsigned stdc_leading_ones_ui(unsigned) noexcept { return 0xBC; }
+unsigned long stdc_leading_ones_ul(unsigned long) noexcept { return 0xBD; }
+unsigned long long stdc_leading_ones_ull(unsigned long long) noexcept {
+  return 0xBF;
+}
+}
+
+#include "include/llvm-libc-macros/stdbit-macros.h"
 
 TEST(LlvmLibcStdbitTest, TypeGenericMacroLeadingZeros) {
   EXPECT_EQ(stdc_leading_zeros(static_cast<unsigned char>(0U)),