[Driver,Darwin] Simplify libc++ include path after #80527. NFC

My #80527 mentioned that `InstalledDir` was weird in the
-canonical-prefixes mode. #70817 was a workaround to find the libc++
include path for a symlinked clang. After #80527, `InstalledDir` was
identical to `Dir` and was subsequently removed, the code change #70817
can be reverted.

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -2514,25 +2514,19 @@ void DarwinClang::AddClangCXXStdlibIncludeArgs(
   switch (GetCXXStdlibType(DriverArgs)) {
   case ToolChain::CST_Libcxx: {
     // On Darwin, libc++ can be installed in one of the following places:
-    // 1. Alongside the compiler in <install>/include/c++/v1
-    // 2. Alongside the compiler in <clang-executable-folder>/../include/c++/v1
-    // 3. In a SDK (or a custom sysroot) in <sysroot>/usr/include/c++/v1
+    // 1. Alongside the compiler in <clang-executable-folder>/../include/c++/v1
+    // 2. In a SDK (or a custom sysroot) in <sysroot>/usr/include/c++/v1
     //
     // The precedence of paths is as listed above, i.e. we take the first path
     // that exists. Note that we never include libc++ twice -- we take the first
     // path that exists and don't send the other paths to CC1 (otherwise
     // include_next could break).
-    //
-    // Also note that in most cases, (1) and (2) are exactly the same path.
-    // Those two paths will differ only when the `clang` program being run
-    // is actually a symlink to the real executable.
 
     // Check for (1)
     // Get from '<install>/bin' to '<install>/include/c++/v1'.
     // Note that InstallBin can be relative, so we use '..' instead of
     // parent_path.
-    llvm::SmallString<128> InstallBin =
-        llvm::StringRef(getDriver().Dir); // <install>/bin
+    llvm::SmallString<128> InstallBin(getDriver().Dir); // <install>/bin
     llvm::sys::path::append(InstallBin, "..", "include", "c++", "v1");
     if (getVFS().exists(InstallBin)) {
       addSystemInclude(DriverArgs, CC1Args, InstallBin);
@@ -2542,20 +2536,7 @@ void DarwinClang::AddClangCXXStdlibIncludeArgs(
                    << "\"\n";
     }
 
-    // (2) Check for the folder where the executable is located, if different.
-    if (getDriver().Dir != getDriver().Dir) {
-      InstallBin = llvm::StringRef(getDriver().Dir);
-      llvm::sys::path::append(InstallBin, "..", "include", "c++", "v1");
-      if (getVFS().exists(InstallBin)) {
-        addSystemInclude(DriverArgs, CC1Args, InstallBin);
-        return;
-      } else if (DriverArgs.hasArg(options::OPT_v)) {
-        llvm::errs() << "ignoring nonexistent directory \"" << InstallBin
-                     << "\"\n";
-      }
-    }
-
-    // Otherwise, check for (3)
+    // Otherwise, check for (2)
     llvm::SmallString<128> SysrootUsr = Sysroot;
     llvm::sys::path::append(SysrootUsr, "usr", "include", "c++", "v1");
     if (getVFS().exists(SysrootUsr)) {