[clangd] Adjust compile flags so they work when applied to other file(type)s.

It's reasonable to want to use the command from one file to compile another.
In particular, the command from a translation unit to parse a related header:
  {"file": "foo.h", "command": "clang foo.cpp"}

This is largely what InterpolatingCompilationDatabase tries to do.
To do this correctly can require nontrivial changes to the argv, because the
file extension affects semantics.  e.g. here we must add "-x c++header".

When external tools compile commands for different files, we should apply the
same adjustments. This is better than telling people to "fix their tools":
 - simple e.g. python scripts shouldn't have to interpret clang argv
 - this is a good way to represent the intent "parse header X in the context of
   file Y", which can work even if X is not self-contained. clangd does not
   support this today, but some other tools do, and we may one day.

This issue is discussed in https://github.com/clangd/clangd/issues/519

Differential Revision: https://reviews.llvm.org/D116167