[clang] [Coverage] Fix assertion failure when a -isystem header invokes a user macro (PR #195427)

[via cfe-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 53769 tests passed
* 1173 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### Clang
<details>
<summary>Clang.CoverageMapping/system_macro_switch.cpp</summary>

```
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 4
rm -rf C:\_work\llvm-project\llvm-project\build\tools\clang\test\CoverageMapping\Output\system_macro_switch.cpp.tmp && split-file C:\_work\llvm-project\llvm-project\clang\test\CoverageMapping\system_macro_switch.cpp C:\_work\llvm-project\llvm-project\build\tools\clang\test\CoverageMapping\Output\system_macro_switch.cpp.tmp && cd C:\_work\llvm-project\llvm-project\build\tools\clang\test\CoverageMapping\Output\system_macro_switch.cpp.tmp
# executed command: rm -rf 'C:\_work\llvm-project\llvm-project\build\tools\clang\test\CoverageMapping\Output\system_macro_switch.cpp.tmp'
# note: command had no output on stdout or stderr
# executed command: split-file 'C:\_work\llvm-project\llvm-project\clang\test\CoverageMapping\system_macro_switch.cpp' 'C:\_work\llvm-project\llvm-project\build\tools\clang\test\CoverageMapping\Output\system_macro_switch.cpp.tmp'
# note: command had no output on stdout or stderr
# executed command: cd 'C:\_work\llvm-project\llvm-project\build\tools\clang\test\CoverageMapping\Output\system_macro_switch.cpp.tmp'
# note: command had no output on stdout or stderr
# RUN: at line 5
c:\_work\llvm-project\llvm-project\build\bin\clang.exe -cc1 -internal-isystem C:\_work\llvm-project\llvm-project\build\lib\clang\23\include -nostdsysteminc -mllvm -emptyline-comment-coverage=false -fprofile-instrument=clang -fcoverage-mapping -dump-coverage-mapping -emit-llvm-only -isystem sys a.cpp | c:\_work\llvm-project\llvm-project\build\bin\filecheck.exe C:\_work\llvm-project\llvm-project\clang\test\CoverageMapping\system_macro_switch.cpp
# executed command: 'c:\_work\llvm-project\llvm-project\build\bin\clang.exe' -cc1 -internal-isystem 'C:\_work\llvm-project\llvm-project\build\lib\clang\23\include' -nostdsysteminc -mllvm -emptyline-comment-coverage=false -fprofile-instrument=clang -fcoverage-mapping -dump-coverage-mapping -emit-llvm-only -isystem sys a.cpp
# note: command had no output on stdout or stderr
# executed command: 'c:\_work\llvm-project\llvm-project\build\bin\filecheck.exe' 'C:\_work\llvm-project\llvm-project\clang\test\CoverageMapping\system_macro_switch.cpp'
# .---command stderr------------
# | C:\_work\llvm-project\llvm-project\clang\test\CoverageMapping\system_macro_switch.cpp:7:17: error: CHECK-LABEL: expected string not found in input
# | // CHECK-LABEL: _ZL3fooi:
# |                 ^
# | <stdin>:1:1: note: scanning from here
# | main:
# | ^
# | <stdin>:3:8: note: possible intended match here
# | <unknown>:?foo@@YAXH at Z:
# |        ^
# | 
# | Input file: <stdin>
# | Check file: C:\_work\llvm-project\llvm-project\clang\test\CoverageMapping\system_macro_switch.cpp
# | 
# | -dump-input=help explains the following input dump.
# | 
# | Input was:
# | <<<<<<
# |            1: main: 
# | label:7'0     X~~~~~ error: no match found
# |            2:  File 0, 9:29 -> 9:43 = #0 
# | label:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |            3: <unknown>:?foo@@YAXH at Z: 
# | label:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~
# | label:7'1            ?                 possible intended match
# |            4:  File 0, 1:24 -> 7:2 = #0 
# | label:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~
# |            5:  Branch,File 0, 2:11 -> 2:12 = ((#2 + #3) + #4), (((#0 - #2) - #3) - #4) 
# | label:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |            6:  Gap,File 0, 2:14 -> 5:10 = 0 
# | label:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |            7:  File 0, 4:21 -> 5:10 = ((#2 + #3) + #4) 
# | label:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |            8:  File 0, 4:21 -> 5:10 = (#2 + #3) 
# | label:7'0     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# |            .
# |            .
# |            .
# | >>>>>>
# `-----------------------------
# error: command failed with exit status: 1

--

```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/195427