[Clang] Missing `#embed` file fails with option `-MG`, unlike GCC

[fuhsnn]

For file foo.c like this:

char arr[] {
#embed "bar"
};

clang foo.c -MM -MG fails with "file not found";
gcc foo.c -MM -MG succeeds and prints foo.o: foo.c bar.
Looks like GCC's -MG treat missing #embed file as missing dependency like missing #include files.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Hsiang-Ying Fu (fuhsnn)

For file `foo.c` like this: ```C char arr[] { #embed "bar" }; ``` `clang foo.c -MM -MG` fails with "file not found"; `gcc foo.c -MM -MG` succeeds and prints `foo.o: foo.c bar`. Looks like GCC's `-MG` treat missing #embed file as missing dependency like missing #include files.

[shafik]

CC @AaronBallman @Fznamznon