[sanitizer] Add "mainfile" prefix to sanitizer special case list

When an issue exists in the main file (caller) instead of an included file
(callee), using a `src` pattern applying to the included file may be
inappropriate if it's the caller's responsibility. Add `mainfile` prefix to check
the main filename.

For the example below, the issue may reside in a.c (foo should not be called
with a misaligned pointer or foo should switch to an unaligned load), but with
`src` we can only apply to the innocent callee a.h. With this patch we can use
the more appropriate `mainfile:a.c`.
```
//--- a.h
// internal linkage
static inline int load(int *x) { return *x; }

//--- a.c, -fsanitize=alignment
#include "a.h"
int foo(void *x) { return load(x); }
```

See the updated clang/docs/SanitizerSpecialCaseList.rst for a caveat due
to C++ vague linkage functions.

Reviewed By: #sanitizers, kstoimenov, vitalybuka

Differential Revision: https://reviews.llvm.org/D129832