[clangd] Make TestTU build with preamble, and fix the fallout.

Our testing didn't reflect reality: live clangd almost always uses a
preamble, and sometimes the preamble behaves differently.
This patch fixes a common test helper to be more realistic.

Preamble doesn't preserve information about which tokens come from the
command-line (this gets inlined into a source file). So remove logic
that attempts to treat symbols with such names differently.

A SymbolCollectorTest tries to verify that locals in headers are not
indexed, with preamble enabled this is only meaningful for locals of
auto-typed functions (otherwise the bodies aren't parsed).

Tests were relying on the fact that the findAnyDecl helper actually did expose
symbols from headers. Resolve by making all these functions consistently
able to find symbols in headers/preambles.

llvm-svn: 346488