[clang] Add support for sysroot-relative system header search paths (PR #82084)

[Huaqi Fang via cfe-commits]

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@@ -3191,6 +3191,22 @@ static bool ParseHeaderSearchArgs(HeaderSearchOptions &Opts, ArgList &Args,
   bool IsIndexHeaderMap = false;
   bool IsSysrootSpecified =
       Args.hasArg(OPT__sysroot_EQ) || Args.hasArg(OPT_isysroot);
+
+  // Expand a leading `=` to the sysroot if one was passed (and it's not a
+  // framework flag).
+  auto PrefixHeaderPath = [IsSysrootSpecified,
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fanghuaqi wrote:

Hello, I have a question about why `IsSysrootSpecified` is required, since the sysroot could use default sysroot defined in toolchain, for gcc, it is not required.

`If dir begins with ‘=’ or $SYSROOT, then the ‘=’ or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.`

So it will replace sysroot defined in toolchain, as tested below:

- gcc `-isystem=/include/libncrt`: `xxxxx/gcc/bin/../riscv64-unknown-elf/include/libncrt`, this is expected
- clang `-isystem=/include/libncrt`  : ignoring nonexistent directory "=/include/libncrt"


https://github.com/llvm/llvm-project/pull/82084