[libc] Fix quotation marks in overlay mode instructions

These were using ” ("right double quotation mark") instead of
the usual ". This means CMake took the value literally and you
got:
```
CMake Error at CMakeLists.txt:139 (MESSAGE):
  ”libc” isn't a known project:
  bolt;clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;lld;lldb;mlir;openmp;polly;pstl;flang.
  Did you mean to enable it as a runtime in LLVM_ENABLE_RUNTIMES?
```
`="libc"` works fine.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D143254