rust: Use libc++ runtime when using clang with llvm runtime

meta-clang has options when it comes to C++ runtime, default is to use
gnu runtime, other options are llvm runtime and android runtime. This
patch helps when a distro is using llvm runtime for C/C++ runtime. It
informs the rust build system about right C++ runtime to configure for
when such a setting is used.

(From OE-Core rev: 521872ab2cac092c88446730772cb8c4e22b0cdc)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/rust/rust.inc

@@ -110,6 +110,8 @@ python do_configure() {
     # [llvm]
     config.add_section("llvm")
     config.set("llvm", "static-libstdcpp", e(False))
+    if "llvm" in (d.getVar('TC_CXX_RUNTIME') or ""):
+        config.set("llvm", "use-libcxx", e(True))
 
     # [rust]
     config.add_section("rust")