[clang] [clang][modules] Headers meant to be included multiple times can be completely invisible in clang module builds (PR #83660)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

Most of the increase seems to be coming from module maps that describe textual headers:
```
// This describes builtin headers.
Before: gen-crosstool-x86.cppmap:1:1: note: file entered 630 times using 83989080B of space
After: gen-crosstool-x86.cppmap:1:1: note: file entered 2260 times using 319523320B of space
```
I will dig further to see where it's coming from.

https://github.com/llvm/llvm-project/pull/83660