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

[Jeremy Wong via cfe-commits]

jrmwng wrote:

@jansvoboda11 the same error using the above steps

```
547>------ Build started: Project: check-all, Configuration: Release x64 ------
547>Running all regression tests
547>llvm-lit.py: C:\GitHub\llvm-project\llvm\utils\lit\lit\llvm\config.py:46: note: using lit tools: C:\Program Files\Git\usr\bin
547>llvm-lit.py: C:\GitHub\llvm-project\llvm\utils\lit\lit\llvm\config.py:488: note: using clang: c:\github\llvm-project\build\release\bin\clang.exe
547>-- Testing: 70944 tests, 32 workers --
547>FAIL: Clang :: Lexer/case-insensitive-include-absolute.c (11482 of 70944)
547>******************** TEST 'Clang :: Lexer/case-insensitive-include-absolute.c' FAILED ********************
547>Exit Code: 2
547>
547>Command Output (stdout):
547>--
547># RUN: at line 3
547>rm -rf C:\GitHub\llvm-project\build\tools\clang\test\Lexer\Output\case-insensitive-include-absolute.c.tmp && split-file C:\GitHub\llvm-project\clang\test\Lexer\case-insensitive-include-absolute.c C:\GitHub\llvm-project\build\tools\clang\test\Lexer\Output\case-insensitive-include-absolute.c.tmp
547># executed command: rm -rf 'C:\GitHub\llvm-project\build\tools\clang\test\Lexer\Output\case-insensitive-include-absolute.c.tmp'
547># executed command: split-file 'C:\GitHub\llvm-project\clang\test\Lexer\case-insensitive-include-absolute.c' 'C:\GitHub\llvm-project\build\tools\clang\test\Lexer\Output\case-insensitive-include-absolute.c.tmp'
547># RUN: at line 4
547>sed "s|DIR|C:/GitHub/llvm-project/build/tools/clang/test/Lexer/Output/case-insensitive-include-absolute.c.tmp|g" C:\GitHub\llvm-project\build\tools\clang\test\Lexer\Output\case-insensitive-include-absolute.c.tmp/tu.c.in > C:\GitHub\llvm-project\build\tools\clang\test\Lexer\Output\case-insensitive-include-absolute.c.tmp/tu.c
547># executed command: sed 's|DIR|C:/GitHub/llvm-project/build/tools/clang/test/Lexer/Output/case-insensitive-include-absolute.c.tmp|g' 'C:\GitHub\llvm-project\build\tools\clang\test\Lexer\Output\case-insensitive-include-absolute.c.tmp/tu.c.in'
547># RUN: at line 5
547>c:\github\llvm-project\build\release\bin\clang.exe -cc1 -internal-isystem F:\GitHub\llvm-project\build\Release\lib\clang\18\include -nostdsysteminc -fsyntax-only C:\GitHub\llvm-project\build\tools\clang\test\Lexer\Output\case-insensitive-include-absolute.c.tmp/tu.c 2>&1 | c:\github\llvm-project\build\release\bin\filecheck.exe C:\GitHub\llvm-project\clang\test\Lexer\case-insensitive-include-absolute.c --DDIR=C:/GitHub/llvm-project/build/tools/clang/test/Lexer/Output/case-insensitive-include-absolute.c.tmp
547># executed command: 'c:\github\llvm-project\build\release\bin\clang.exe' -cc1 -internal-isystem 'F:\GitHub\llvm-project\build\Release\lib\clang\18\include' -nostdsysteminc -fsyntax-only 'C:\GitHub\llvm-project\build\tools\clang\test\Lexer\Output\case-insensitive-include-absolute.c.tmp/tu.c'
547># executed command: 'c:\github\llvm-project\build\release\bin\filecheck.exe' 'C:\GitHub\llvm-project\clang\test\Lexer\case-insensitive-include-absolute.c' --DDIR=C:/GitHub/llvm-project/build/tools/clang/test/Lexer/Output/case-insensitive-include-absolute.c.tmp
547># .---command stderr------------
547>CUSTOMBUILD : # | FileCheck error : '<stdin>' is empty.
547># | FileCheck command line:  c:\github\llvm-project\build\release\bin\filecheck.exe C:\GitHub\llvm-project\clang\test\Lexer\case-insensitive-include-absolute.c --DDIR=C:/GitHub/llvm-project/build/tools/clang/test/Lexer/Output/case-insensitive-include-absolute.c.tmp
547># `-----------------------------
547>CUSTOMBUILD : # error : command failed with exit status: 2
547>
547>--
547>
547>********************
547>********************
547>Failed Tests (1):
547>  Clang :: Lexer/case-insensitive-include-absolute.c
547>
547>
547>Testing Time: 433.52s
547>
547>Total Discovered Tests: 92696
547>  Skipped          :    90 (0.10%)
547>  Unsupported      : 24219 (26.13%)
547>  Passed           : 68283 (73.66%)
547>  Expectedly Failed:   103 (0.11%)
547>  Failed           :     1 (0.00%)
547>C:\Program Files\Microsoft Visual Studio\2022\Community\MSBuild\Microsoft\VC\v170\Microsoft.CppCommon.targets(254,5): error MSB8066: Custom build for 'C:\GitHub\llvm-project\build\CMakeFiles\5344a242bf0d5667ee3e0d3564612537\check-all.rule;C:\GitHub\llvm-project\llvm\CMakeLists.txt' exited with code 1.
547>Done building project "check-all.vcxproj" -- FAILED.
```

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