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

[Huaqi Fang 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,
----------------
fanghuaqi 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`?

I found it is caused by this https://github.com/llvm/llvm-project/blob/53c785d8595f42a47ecc2f0f09d8315b10edfb79/clang/lib/Driver/ToolChains/Clang.cpp#L1084-L1092

The `sysroot` returned is empty, but actually it can be computed like this `getToolChain().computeSysRoot()`, for my case, it is done in https://github.com/llvm/llvm-project/blob/53c785d8595f42a47ecc2f0f09d8315b10edfb79/clang/lib/Driver/ToolChains/BareMetal.cpp#L165

And locally I modified the code `clang/lib/Driver/ToolChains/Clang.cpp` in my side like this, then it fixed my issue.
~~~diff
   // If we have a --sysroot, and don't have an explicit -isysroot flag, add an
   // -isysroot to the CC1 invocation.
-  StringRef sysroot = C.getSysRoot();
-  if (sysroot != "") {
-    if (!Args.hasArg(options::OPT_isysroot)) {
-      CmdArgs.push_back("-isysroot");
-      CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
-    }
+  std::string sysroot = std::string(C.getSysRoot());
+  if (sysroot.empty()) {
+    sysroot = getToolChain().computeSysRoot();
+  }
+  if (!Args.hasArg(options::OPT_isysroot)) {
+    CmdArgs.push_back("-isysroot");
+    CmdArgs.push_back(C.getArgs().MakeArgString(sysroot));
   }
~~~

Regarding `Are you specifically asking to add a fallback to `<clang>/../<target>/include`?`

I think it will be good, since the `=` is not handled correctly, it directly returned invalid path `=/include/libncrt` not a sysroot path + `/include/libncrt`

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