[libc] Remove GPU string functions incompatible with C++

[jhuber6]

Summary

These functions have definitions differing between C and C++. GNU
respects the C++ definitions while the LLVM libc does not. This causes
many bugs and the current hack creates other issues. Rather than hack
around this I'd rather temporarily disable these than regress with the
integration into other offloading languages. We lose test support for
them but we should be able to re-enable these once the libc headers
provide these correctly.

[JonChesterfield]

Is this a const thing? If so I'd naively hope we can declare both in C++ mode and alias them, but I can believe that is detectably broken.

How does glibc manage their hack?

clang/lib/Headers/llvm_libc_wrappers/string.h

 //===-- Wrapper for C standard string.h declarations on the GPU -----------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef __CLANG_LLVM_LIBC_WRAPPERS_STRING_H__
 #define __CLANG_LLVM_LIBC_WRAPPERS_STRING_H__
 
 #if !defined(_OPENMP) && !defined(__HIP__) && !defined(__CUDA__)
 #error "This file is for GPU offloading compilation only"
 #endif
 
-// The GNU headers provide non C-standard headers when in C++ mode. Manually
-// undefine it here so that the definitions agree with the C standard for our
-// purposes.
-#ifdef __cplusplus
-extern "C" {
-#pragma push_macro("__cplusplus")
-#undef __cplusplus
-#endif
-
+// FIXME: The GNU headers provide C++ standard compliant headers when in C++
+// mode and the LLVM libc does not. We cannot enable memchr, strchr, strchrnul,
+// strpbrk, strrchr, strstr, or strcasestr until this is addressed.
 #include_next <string.h>
 
-#pragma pop_macro("__cplusplus")
-#ifdef __cplusplus
-}
-#endif
-
 #if __has_include(<llvm-libc-decls/string.h>)
 
 #if defined(__HIP__) || defined(__CUDA__)
 #define __LIBC_ATTRS __attribute__((device))
 #endif
 
 #pragma omp begin declare target
 

libc/config/gpu/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.ctype.toascii
     libc.src.ctype.tolower
     libc.src.ctype.toupper
 
     # string.h entrypoints
     libc.src.string.bcmp
     libc.src.string.bzero
     libc.src.string.memccpy
-    libc.src.string.memchr
     libc.src.string.memcmp
     libc.src.string.memcpy
     libc.src.string.memmem
     libc.src.string.memmove
     libc.src.string.mempcpy
     libc.src.string.memrchr
     libc.src.string.memset
     libc.src.string.stpcpy
     libc.src.string.stpncpy
     libc.src.string.strcasecmp
-    libc.src.string.strcasestr
     libc.src.string.strcat
-    libc.src.string.strchr
-    libc.src.string.strchrnul
     libc.src.string.strcmp
     libc.src.string.strcpy
     libc.src.string.strcspn
     libc.src.string.strlcat
     libc.src.string.strlcpy
     libc.src.string.strlen
     libc.src.string.strncasecmp
     libc.src.string.strncat
     libc.src.string.strncmp
     libc.src.string.strncpy
     libc.src.string.strnlen
-    libc.src.string.strpbrk
-    libc.src.string.strrchr
     libc.src.string.strspn
-    libc.src.string.strstr
     libc.src.string.strtok
     libc.src.string.strtok_r
 
     # stdlib.h entrypoints
     libc.src.stdlib.abs
     libc.src.stdlib.atoi
     libc.src.stdlib.atof
     libc.src.stdlib.atol

libc/docs/gpu/support.rst

 --------
 
 =============  =========  ============
 Function Name  Available  RPC Required
 =============  =========  ============
 bcmp           |check|
 bzero          |check|
 memccpy        |check|
-memchr         |check|
+memchr         
 memcmp         |check|
 memcpy         |check|
 memmove        |check|
 mempcpy        |check|
 memrchr        |check|
 memset         |check|
 stpcpy         |check|
 stpncpy        |check|
 strcat         |check|
-strchr         |check|
+strchr         
 strcmp         |check|
 strcpy         |check|
 strcspn        |check|
 strlcat        |check|
 strlcpy        |check|
 strlen         |check|
 strncat        |check|
 strncmp        |check|
 strncpy        |check|
 strnlen        |check|
-strpbrk        |check|
-strrchr        |check|
+strpbrk        
+strrchr        
 strspn         |check|
-strstr         |check|
+strstr         
 strtok         |check|
 strtok_r       |check|
 strdup
 strndup
 =============  =========  ============
 
 stdlib.h
 --------