[clang-tools-extra] `clangd`: support `-stdlib=` flags from `compile_commands.json`. (PR #69283)

[Nathan Ridge via cfe-commits]

================
@@ -203,6 +214,7 @@ template <> struct DenseMapInfo<DriverArgs> {
         Val.Driver,
         Val.StandardIncludes,
         Val.StandardCXXIncludes,
+        Val.Stdlib,
----------------
HighCommander4 wrote:

nit: it looks like the previous patch which introduced `Target` forgot to add it here, could you add please?

https://github.com/llvm/llvm-project/pull/69283