[Driver] Detect libstdc++ include paths for native gcc (-m32 and -m64) on Debian...

[Driver] Detect libstdc++ include paths for native gcc (-m32 and -m64) on Debian i386

Take gcc-8 on Debian i386 as an example. The target-specific libstdc++ search
path (`GPLUSPLUS_TOOL_INCLUDE_DIR`) uses the multiarch name `i386-linux-gnu`,
instead of the triple of the GCC installation `i686-linux-gnu` (the directory
under `usr/lib/gcc/`):

```
/usr/include/c++/8
/usr/include/i386-linux-gnu/c++/8
/usr/include/c++/8/backward
```

Clang currently detects `/usr/lib/gcc/i686-linux-gnu/8/../../../include/i686-linux-gnu/c++/8`.
This patch changes the second i686-linux-gnu to i386-linux-gnu so that
`/usr/include/i386-linux-gnu/c++/8` can be found.

Fix PR49827 - this was somehow regressed by my previous libstdc++ include path
cleanups and fixes for gcc-cross, but it seems that the paths were never properly tested before.

Differential Revision: https://reviews.llvm.org/D99852