[Docs] Fix minor issues in AdvancedBuilds documentation

[aidengrossman]

Summary

This patch modifies the commands under the Bootstrap builds section so
that they include the -DLLVM_ENABLE_PROJECTS flag with the value "clang"
so that the Clang build will actually get picked up. Without this patch
they error out as the CLANG_BOOTSTRAP_PASSTHROUGH variable is processed
in /clang/CMakeLists.txt which isn't picked up without the
LLVM_ENABLE_PROJECTS variable being set appropriately.

This patch also changes any remaining dangling <path to source>
references to <path to source>/llvm to better match the rest of the
file.

[aidengrossman]

Based on some discussion that happened on Discord where someone has having trouble getting a multi-stage build to work using this documentation as the LLVM_ENABLE_PROJECTS variable wasn't set.

[thieta]

LGTM thanks!

llvm/docs/AdvancedBuilds.rst

 
 In a simple two-stage bootstrap build, we build clang using the system compiler,
 then use that just-built clang to build clang again. In CMake this simplest form
 of a bootstrap build can be configured with a single option,
 CLANG_ENABLE_BOOTSTRAP.
 
 .. code-block:: console
 
-  $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCLANG_ENABLE_BOOTSTRAP=On <path to source>
+  $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
+      -DCLANG_ENABLE_BOOTSTRAP=On \
+      -DLLVM_ENABLE_PROJECTS="clang" \
+      <path to source>/llvm
   $ ninja stage2
 
 This command itself isn't terribly useful because it assumes default
 configurations for each stage. The next series of examples utilize CMake cache
 scripts to provide more complex options.
 
 By default, only a few CMake options will be passed between stages.
 The list, called _BOOTSTRAP_DEFAULT_PASSTHROUGH, is defined in clang/CMakeLists.txt.
 To force the passing of the variables between stages, use the -DCLANG_BOOTSTRAP_PASSTHROUGH
 CMake option, each variable separated by a ";". As example:
 
 .. code-block:: console
 
-  $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release -DCLANG_ENABLE_BOOTSTRAP=On -DCLANG_BOOTSTRAP_PASSTHROUGH="CMAKE_INSTALL_PREFIX;CMAKE_VERBOSE_MAKEFILE" <path to source>
+  $ cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
+      -DCLANG_ENABLE_BOOTSTRAP=On \
+      -DCLANG_BOOTSTRAP_PASSTHROUGH="CMAKE_INSTALL_PREFIX;CMAKE_VERBOSE_MAKEFILE" \
+      -DLLVM_ENABLE_PROJECTS="clang" \
+      <path to source>/llvm
   $ ninja stage2
 
 CMake options starting by ``BOOTSTRAP_`` will be passed only to the stage2 build.
 This gives the opportunity to use Clang specific build flags.
 For example, the following CMake call will enabled '-fno-addrsig' only during
 the stage2 build for C and C++.
 
 .. code-block:: console

 The stage2 compiler is the fully optimized compiler intended to ship to users.
 
 Setting up these compilers requires a lot of options. To simplify the
 configuration the Apple Clang build settings are contained in CMake Cache files.
 You can build an Apple Clang compiler using the following commands:
 
 .. code-block:: console
 
-  $ cmake -G Ninja -C <path to source>/clang/cmake/caches/Apple-stage1.cmake <path to source>
+  $ cmake -G Ninja -C <path to source>/clang/cmake/caches/Apple-stage1.cmake <path to source>/llvm
   $ ninja stage2-distribution
 
 This CMake invocation configures the stage1 host compiler, and sets
 CLANG_BOOTSTRAP_CMAKE_ARGS to pass the Apple-stage2.cmake cache script to the
 stage2 configuration step.
 
 When you build the stage2-distribution target it builds the minimal stage1
 compiler and required tools, then configures and builds the stage2 compiler

 this, you have a stage2 and stage3 compiler that should be bit-for-bit
 identical.
 
 You can perform one of these 3-stage builds with LLVM & clang using the
 following commands:
 
 .. code-block:: console
 
-  $ cmake -G Ninja -C <path to source>/clang/cmake/caches/3-stage.cmake <path to source>
+  $ cmake -G Ninja -C <path to source>/clang/cmake/caches/3-stage.cmake <path to source>/llvm
   $ cmake --build . --target stage3 --parallel
 
 After the build you can compare the stage2 & stage3 compilers.