[Driver] Remove unused input type code (NFCI)

Summary:
When the Swift driver executable was first added in
ed20385,
the code included a FIXME to support input types, like Clang's
`-x <language>` option. Perhaps as a first step in implementing this
functionality, it also included dead code that manipulated an input type
argument, even though the variable for storing the argument was never
written to.

Remove the unused input type code, and the FIXME. The FIXME is now
tracked with a Swift bug URL, so that a discussion on next steps can be had:
https://bugs.swift.org/browse/SR-6054

Test plan: `utils/build-script --release --test`

Author: modocache

include/swift/Driver/ToolChain.h

@@ -167,6 +167,8 @@ class ToolChain {
                                     const OutputInfo &OI) const;
 
   /// Return the default language type to use for the given extension.
+  /// If the extension is empty or is otherwise not recognized, return
+  /// the invalid type \c TY_INVALID.
   virtual types::ID lookupTypeForExtension(StringRef Ext) const;
 
   /// Check whether a clang library with a given name exists.

include/swift/Driver/Types.h

@@ -36,6 +36,8 @@ namespace types {
   StringRef getTypeTempSuffix(ID Id);
 
   /// Lookup the type to use for the file extension \p Ext.
+  /// If the extension is empty or is otherwise not recognized, return
+  /// the invalid type \c TY_INVALID.
   ID lookupTypeForExtension(StringRef Ext);
 
   /// Lookup the type to use for the name \p Name.

lib/Driver/Driver.cpp

@@ -926,39 +926,26 @@ static bool checkInputExistence(const Driver &D, const DerivedArgList &Args,
 void Driver::buildInputs(const ToolChain &TC,
                          const DerivedArgList &Args,
                          InputFileList &Inputs) const {
-  types::ID InputType = types::TY_Nothing;
-  Arg *InputTypeArg = nullptr;
-
   llvm::StringMap<StringRef> SourceFileNames;
 
   for (Arg *A : Args) {
     if (A->getOption().getKind() == Option::InputClass) {
       StringRef Value = A->getValue();
       types::ID Ty = types::TY_INVALID;
 
-      if (InputType == types::TY_Nothing) {
-        // If there was an explicit arg for this, claim it.
-        if (InputTypeArg)
-          InputTypeArg->claim();
-
-        // stdin must be handled specially.
-        if (Value.equals("-")) {
-          // By default, treat stdin as Swift input.
-          // FIXME: should we limit this inference to specific modes?
-          Ty = types::TY_Swift;
-        } else {
-          // Otherwise lookup by extension.
-          Ty = TC.lookupTypeForExtension(llvm::sys::path::extension(Value));
+      // stdin must be handled specially.
+      if (Value.equals("-")) {
+        // By default, treat stdin as Swift input.
+        Ty = types::TY_Swift;
+      } else {
+        // Otherwise lookup by extension.
+        Ty = TC.lookupTypeForExtension(llvm::sys::path::extension(Value));
 
-          if (Ty == types::TY_INVALID) {
-            // FIXME: should we adjust this inference in certain modes?
-            Ty = types::TY_Object;
-          }
+        if (Ty == types::TY_INVALID) {
+          // By default, treat inputs with no extension, or with an
+          // extension that isn't recognized, as object files.
+          Ty = types::TY_Object;
         }
-      } else {
-        assert(InputTypeArg && "InputType set w/o InputTypeArg");
-        InputTypeArg->claim();
-        Ty = InputType;
       }
 
       if (checkInputExistence(*this, Args, Diags, Value))
@@ -973,8 +960,6 @@ void Driver::buildInputs(const ToolChain &TC,
         }
       }
     }
-
-    // FIXME: add -x support (or equivalent)
   }
 }