Make most clangd unittests pass on Windows

The Windows triple currently turns on delayed template parsing, which
confuses several unit tests that use templates.

For now, just explicitly disable delayed template parsing. This isn't
ideal, but:

- the Windows triple will soon no longer use delayed template parsing
  by default

- there's precedent for this in the clangd unit tests already

- let's get the clangd tests pass on Windows first before making
  behavioral changes

Part of PR43592.

llvm-svn: 374718