[Driver] Detect libstdc++ include paths for native gcc (-m32 and -m64) on Debian i386Take gcc-8 on Debian i386 as an example. The target-specific libstdc++ searchpath (`GPLUSPLUS_TOOL_INCLUDE_DIR`
[Driver] Detect libstdc++ include paths for native gcc (-m32 and -m64) on Debian i386Take gcc-8 on Debian i386 as an example. The target-specific libstdc++ searchpath (`GPLUSPLUS_TOOL_INCLUDE_DIR`) uses the multiarch name `i386-linux-gnu`,instead of the triple of the GCC installation `i686-linux-gnu` (the directoryunder `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 pathcleanups and fixes for gcc-cross, but it seems that the paths were never properly tested before.Differential Revision: https://reviews.llvm.org/D99852
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