[Coverage] Fix assertion failure when a -isystem header invokes a user macro (#195427)
```
// a.cc
static void foo(int x) {
switch (x) {
#define GENERIC(n) case n:
#include "types.def" // -isystem header invokes a user macro
break;
}
}
// sys/types.def
#define MID(name) GENERIC(name)
MID(0)
MID(1)
MID(2)
```
```
$ clang -fprofile-instr-generate -fcoverage-mapping -isystem sys -c a.cc
Assertion `SystemHeadersCoverage ||
!SM.isInSystemHeader(SM.getSpellingLoc(Loc))' failed.
```
Commit 702a2b627ff4 ("[Coverage] Rework !SystemHeadersCoverage")
replaced the system-header skip in gatherFileIDs with this assertion,
which trips as `SM.isInSystemHeader(SM.getSpellingLoc(Loc))` is false.
This patch adds back the pre-#91446 condition but folds it with
the macro-token remap `if` statement.
Fixes #179316/#195422.
Clang Opus 4.7 identified clang/lib/Parse/ParseExpr.cpp, created a
minimal reproduce with cvise, and wrote the initial version of this
CodeGen patch. (An earlier session papered over the bug by patching
llvm-cov instead, which I abandoned).
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