[clang] [clang][lex] Fix non-portability diagnostics with absolute path (PR #74782)

[Jeremy Wong via cfe-commits]

jrmwng wrote:

@jansvoboda11 it should be the following steps. (I try to repeat it now)

1. CMake (cmake-gui)
    - Where is the source code: C:/GitHub/llvm-project/
    - Where to build the binaries: C:/GitHub/llvm-project/build
    - Click the "Configure" button
      - Create Directory --> "Yes"
      - Specify the generator for this project: Visual Studio 17 2022
      - Optional platform for generator: x64
      - Optional toolset to use: ClangCL
      - Use default native compilers
      - Click the "Finish" button
    - Modify the following entries
      - LLVM_ENABLE_PROJECTS: clang
      - LLVM_TARGETS_TO_BUILD: X86;AArch64
    - Click the "Configure" button again
    - Click the "Generate" button
    - Click the "Open Project" button
2. Visual Studio is opened
    - Change configuration from "Debug" to "Release"
    - Build the "check-all" project


https://github.com/llvm/llvm-project/pull/74782