clang++ Doesn't Select The Correct Include Directories When Cross-Compiling

[Connor Nolan]

## What Happens?

When cross-compiling with clang++, it isn't able to find the correct C++ headers and fails to compile. Meanwhile, arm-linux-gnueabihf-g++ compiles fine.

## Test Code

test.cpp:
> #include <string>
> #include <cstdio>
>
> int main() {
> std::string str = "Hello World!";
> printf("%s\n", str.c_str());
> }

## Compiling With GCC

$ arm-linux-gnueabihf-g++ -o test test.cpp
$ echo $?
0
$ file test
test: ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, BuildID[sha1]=e0b6f7f2bec2ca44794a814775f6587aecaf3c5d, for GNU/Linux 3.2.0, not stripped

## Compiling With Clang

$ clang++ -target arm-linux-gnueabihf -o test test.cpp
In file included from test.cpp:1:
/usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10/../../../../include/c++/10/string:38:10: fatal error: 'bits/c++config.h' file not found
#include <bits/c++config.h>
         ^~~~~~~~~~~~~~~~~~
1 error generated.

## GCC Include Directories

$ "$(arm-linux-gnueabihf-gcc -print-prog-name=cc1plus)" -v -quiet -imultiarch arm-linux-gnueabihf < /dev/null
ignoring nonexistent directory "/usr/local/include/arm-linux-gnueabihf"
ignoring nonexistent directory "/usr/lib/gcc-cross/arm-linux-gnueabihf/9/include-fixed"
ignoring nonexistent directory "/usr/include/arm-linux-gnueabihf"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc-cross/arm-linux-gnueabihf/9/../../../../arm-linux-gnueabihf/include/c++/9
 /usr/lib/gcc-cross/arm-linux-gnueabihf/9/../../../../arm-linux-gnueabihf/include/c++/9/arm-linux-gnueabihf
 /usr/lib/gcc-cross/arm-linux-gnueabihf/9/../../../../arm-linux-gnueabihf/include/c++/9/backward
 /usr/lib/gcc-cross/arm-linux-gnueabihf/9/include
 /usr/lib/gcc-cross/arm-linux-gnueabihf/9/../../../../arm-linux-gnueabihf/include
 /usr/include
End of search list.

## Clang Include Directories

$ clang++ -target arm-linux-gnueabihf -E -x c++ - -v < /dev/null
clang version 10.0.0-4ubuntu1
Target: arm-unknown-linux-gnueabihf
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10
Found candidate GCC installation: /usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/9
Found candidate GCC installation: /usr/lib/gcc-cross/arm-linux-gnueabihf/10
Found candidate GCC installation: /usr/lib/gcc-cross/arm-linux-gnueabihf/9
Selected GCC installation: /usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10
Candidate multilib: .;@m32
Selected multilib: .;@m32
 (in-process)
 "/usr/lib/llvm-10/bin/clang" -cc1 -triple armv7-unknown-linux-gnueabihf -E -disable-free -disable-llvm-verifier -discard-value-names -main-file-name - -mrelocation-model static -mthread-model posix -mframe-pointer=all -fmath-errno -fno-rounding-math -masm-verbose -mconstructor-aliases -target-cpu generic -target-abi aapcs-linux -mfloat-abi hard -fallow-half-arguments-and-returns -dwarf-column-info -fno-split-dwarf-inlining -debugger-tuning=gdb -v -resource-dir /usr/lib/llvm-10/lib/clang/10.0.0 -internal-isystem /usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10/../../../../include/c++/10 -internal-isystem /usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10/../../../../include/arm-linux-gnueabi/c++/10 -internal-isystem /usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10/../../../../include/arm-linux-gnueabihf/c++/10 -internal-isystem /usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10/../../../../include/c++/10/backward -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm-10/lib/clang/10.0.0/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -fdebug-compilation-dir /home/<User>/Documents/clang-bug -ferror-limit 19 -fmessage-length 0 -fno-signed-char -fgnuc-version=4.2.1 -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -faddrsig -o - -x c++ -
clang -cc1 version 10.0.0 based upon LLVM 10.0.0 default target x86_64-pc-linux-gnu
ignoring nonexistent directory "/usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10/../../../../include/arm-linux-gnueabi/c++/10"
ignoring nonexistent directory "/usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10/../../../../include/arm-linux-gnueabihf/c++/10"
#include "..." search starts here:
#include <...> search starts here:
 /usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10/../../../../include/c++/10
 /usr/bin/../lib/gcc-cross/arm-linux-gnueabihf/10/../../../../include/c++/10/backward
 /usr/local/include
 /usr/lib/llvm-10/lib/clang/10.0.0/include
 /include
 /usr/include
End of search list.
# 1 "<stdin>"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 404 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "<stdin>" 2

## System Information

$ lsb_release -rd
Description: Ubuntu 20.04.3 LTS
Release: 20.04