deps: backport ICU-21081 for ICU 67.x (constexpr)

- Backport of unicode-org/icu@715d254
- ICU bug: https://unicode-org.atlassian.net/browse/ICU-21081

PR-URL: #33337
Refs: https://github.com/unicode-org/icu/releases/tag/release-67-1
Reviewed-By: Steven R Loomis <srloomis@us.ibm.com>

deps/icu-small/README-SMALL-ICU.txt

@@ -3,6 +3,6 @@ Small ICU sources - auto generated by shrink-icu-src.py
 This directory contains the ICU subset used by --with-intl=small-icu (the default)
 It is a strict subset of ICU 67 source files with the following exception(s):
 * deps/icu-small/source/data/in/icudt67l.dat : Reduced-size data file
-
+* deps/icu-small/source/common/uassert.h : Manual backport of ICU-21081
 
 To rebuild this directory, see ../../tools/icu/README.md

deps/icu-small/source/common/uassert.h

@@ -32,7 +32,7 @@
 #   include <assert.h>
 #   define U_ASSERT(exp) assert(exp)
 #elif U_CPLUSPLUS_VERSION
-#   define U_ASSERT(exp) void()
+#   define U_ASSERT(exp) (void)0
 #else
 #   define U_ASSERT(exp)
 #endif