[clang] [Hexagon] Disable system include paths (PR #185456)

[Brian Cain via cfe-commits]

androm3da wrote:

> Hexagon toolchains are almost always cross-compiling and using system include paths is vrtually always an error. In general, adding implicit paths is confusing and a comment in `InitHeaderSearch::AddDefaultIncludePaths()` suggests that "this code path is going away" and the proper place for path selection is in the driver. The current logic is to use system paths by default but it looks like almost all majors OSes are explictly excluded.

*almost* indeed.  Some of us do make native toolchains though 😉

> 
> Disable implicit system include paths for Hexagon targets.

What practical problem are we trying to solve by making this change?

cc @SidManning 

https://github.com/llvm/llvm-project/pull/185456