Provide a custom entry for specifying module cache paths

This is really handy for use in bots with multiple competing workspaces, for
builds that do not provide an explicit module cache. This is controlled
by the CUSTOM_CLANG_MODULE_CACHE env variable.

export CUSTOM_CLANG_MODULE_CACHE="/tmp/yoyo"
$ echo "int foo()" | clang -x objective-c -fmodules -\#\#\# - 2>&1 | tr " " "\n" | grep "\-cache"
"-fmodules-cache-path=/tmp/yoyo"

rdar://problem/48443680

lib/Driver/ToolChains/Clang.cpp

@@ -2651,6 +2651,11 @@ static void RenderBuiltinOptions(const ToolChain &TC, const llvm::Triple &T,
 }
 
 void Driver::getDefaultModuleCachePath(SmallVectorImpl<char> &Result) {
+  if (const char *ModuleCacheStrDir = ::getenv("CUSTOM_CLANG_MODULE_CACHE")) {
+    Result.append(ModuleCacheStrDir,
+                  ModuleCacheStrDir + strlen(ModuleCacheStrDir));
+    return;
+  }
   llvm::sys::path::system_temp_directory(/*erasedOnReboot=*/false, Result);
   llvm::sys::path::append(Result, "org.llvm.clang.");
   appendUserToPath(Result);