[clang][bytecode] Fix __builtin_strchr with unsigned chars

Get the zero-extended truncated desired value in that case.
Add one RUN line to the constexpr-string.cpp test case, to not increase
the runtime of that test too much.

Author: tbaederr

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -2042,10 +2042,16 @@ static bool interp__builtin_memchr(InterpState &S, CodePtr OpPC,
   }
 
   if (ID == Builtin::BIstrchr || ID == Builtin::BI__builtin_strchr) {
+    int64_t DesiredTrunc;
+    if (S.getASTContext().CharTy->isSignedIntegerType())
+      DesiredTrunc =
+          Desired.trunc(S.getASTContext().getCharWidth()).getSExtValue();
+    else
+      DesiredTrunc =
+          Desired.trunc(S.getASTContext().getCharWidth()).getZExtValue();
     // strchr compares directly to the passed integer, and therefore
     // always fails if given an int that is not a char.
-    if (Desired !=
-        Desired.trunc(S.getASTContext().getCharWidth()).getSExtValue()) {
+    if (Desired != DesiredTrunc) {
       S.Stk.push<Pointer>();
       return true;
     }

clang/test/SemaCXX/constexpr-string.cpp

@@ -7,6 +7,8 @@
 // RUN: %clang_cc1 %s -triple armebv7-unknown-linux -std=c++2a -fsyntax-only -verify -pedantic -Wno-vla-extension -fno-signed-char
 // RUN: %clang_cc1 %s -triple armebv7-unknown-linux -std=c++2a -fsyntax-only -verify -pedantic -Wno-vla-extension -fno-wchar -DNO_PREDEFINED_WCHAR_T
 
+// RUN: %clang_cc1 %s -triple armebv7-unknown-linux -std=c++2a -fsyntax-only -verify -pedantic -Wno-vla-extension -fno-signed-char -fexperimental-new-constant-interpreter
+
 # 9 "/usr/include/string.h" 1 3 4  // expected-warning {{this style of line directive is a GNU extension}}
 extern "C" {
   typedef decltype(sizeof(int)) size_t;