Adjust documentation for git migration.

This fixes most references to the paths:

to instead point to

This is *not* a trivial substitution, because additionally, all the
checkout instructions had to be migrated to instruct users on how to
use the monorepo layout, setting LLVM_ENABLE_PROJECTS instead of
checking out various projects into various subdirectories.

I've attempted to not change any scripts here, only documentation. The
scripts will have to be addressed separately.

Additionally, I've deleted one document which appeared to be outdated
and unneeded:

Differential Revision:

llvm-svn: 352514
GitOrigin-RevId: 5d71fc5d7b5ffe2323418a09db6eddaf84d6c662
diff --git a/README.TXT b/README.TXT
index 8fc60cb..c48e855 100644
@@ -38,20 +38,14 @@
 To build and install llgo:
-    # Checkout LLVM:
-    svn co /path/to/llvm
-    # Checkout Clang:
-    cd /path/to/llvm/tools
-    svn co clang
-    # Checkout llgo:
-    svn co llgo
+    # Checkout llvm project.
+    git clone
     # Build LLVM, Clang and llgo: (see also
-    mkdir /path/to/llvm-build
-    cd /path/to/llvm-build
-    cmake /path/to/llvm -DCMAKE_INSTALL_PREFIX=/path/to/llvm-inst
+    cd llvm-project
+    mkdir build
+    cd build
+    cmake ../llvm -DLLVM_ENABLE_PROJECTS='clang;llgo' -DCMAKE_INSTALL_PREFIX=/path/to/llvm-inst
     make install