Wrong README in branch 2.7

[JonathanFoo0523]

README in this branch incorrectly mentioned version 2.9 when the llvm-source code is 2.7.

I verified this is indeed the case by downloading the source code form the release page, and performing diff:

ubuntu@cloud-vm-43-154:~$ diff -r llvm-2.7 llvm-legacy/llvm-2.7/
Only in llvm-2.7/docs/CommandGuide: man
Only in llvm-2.7/docs/CommandGuide: ps
Only in llvm-2.7/test: Debugger
Only in llvm-legacy/llvm-2.7/tools: clang
Only in llvm-2.7/tools: llvm2cpp
Only in llvm-2.7/tools/llvmc/plugins: Hello
Only in llvm-2.7/tools/llvmc/plugins: Simple
Only in llvm-2.7/utils/lit: ExampleTests
Only in llvm-2.7/utils/lit: ExampleTests.ObjDir
Only in llvm-2.7/utils/unittest/googletest: src

There's no different in file content, but some files are present in the downloaded source but not in the branch's source code.

[JonathanFoo0523]

I can confirmed that it can be build from 'gcc 11.4.0`.

It also modifies file cmake/modules/LLVMLibDeps.cmake