Adjust driver references to use the install version. (#4126)

Also makes an alias so that this is easier to find. Verified that
running the alias still finds prelude files.

Note the actual target is printed when building (although a symlink is
also created, using that symlink confuses file-finding).

```
╚╡bazel build :carbon
...
Target //toolchain/install:prefix_root/bin/carbon up-to-date:
  bazel-bin/toolchain/install/prefix_root/bin/carbon
```

README.md

@@ -311,7 +311,7 @@ Carbon:
 
 ```shell
 # Build and run the toolchain's help to get documentation on the command line.
-$ bazel run //toolchain/driver:carbon -- help
+$ bazel run //toolchain -- help
 ```
 
 For complete instructions, including installing dependencies on various

docs/project/contribution_tools.md

@@ -273,13 +273,13 @@ Pass `-c dbg` to `bazel build` in order to compile with debugging enabled. For
 example:
 
 ```shell
-bazel build -c dbg //toolchain/driver:carbon
+bazel build -c dbg //toolchain
 ```
 
 Then debugging works with GDB:
 
 ```shell
-gdb bazel-bin/toolchain/driver/carbon
+gdb bazel-bin/toolchain/install/prefix_root/bin/carbon
 ```
 
 Note that LLVM uses DWARF v5 debug symbols, which means that GDB version 10.1 or
@@ -301,7 +301,7 @@ for more information. To workaround, provide the `--spawn_strategy=local` option
 to Bazel for the debug build, like:
 
 ```shell
-bazel build --spawn_strategy=local -c dbg //toolchain/driver:carbon
+bazel build --spawn_strategy=local -c dbg //toolchain
 ```
 
 You should then be able to debug with `lldb`.

toolchain/BUILD

@@ -0,0 +1,10 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+# A convenience target for running the toolchain with the full prelude
+# available.
+alias(
+    name = "toolchain",
+    actual = "//toolchain/install:prefix_root/bin/carbon",
+)

toolchain/driver/BUILD

@@ -118,6 +118,8 @@ cc_fuzz_test(
     ],
 )
 
+# This target doesn't include prelude libraries. To get a target that has the
+# prelude available, use //toolchain.
 cc_binary(
     name = "carbon",
     srcs = ["driver_main.cpp"],