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

[Jan Svoboda via cfe-commits]

================
@@ -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,
----------------
jansvoboda11 wrote:

My understanding is that with this patch, whenever is sysroot specified we match the GCC behavior (assuming the `=` spelling). Are you specifically asking to add a fallback to `<clang>/../<target>/include`?

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