xds: simplify code handling certain error conditions in the resolver

[easwars]

Simplifies error handling for a couple of cases:

• When constructing a matcher from a Route that has already gone through validation checks in the xDS client, we can ignore errors.
• When building service config for LB policy config that we construct by hand.

Simplifying the handling of these cases greatly simplifies the next task, which is to ensure that any error reported by the resolver to the channel must include the xDS node ID. That change will be made in a follow-up PR.

Addresses: #7931

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 60.86957% with 9 lines in your changes missing coverage. Please review.

Project coverage is 82.32%. Comparing base (aa629e0) to head (074bc1e).

Files with missing lines	Patch %	Lines
xds/internal/resolver/xds_resolver.go	46.66%	6 Missing and 2 partials ⚠️
xds/internal/resolver/serviceconfig.go	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8123      +/-   ##
==========================================
- Coverage   82.36%   82.32%   -0.04%     
==========================================
  Files         387      387              
  Lines       38947    38932      -15     
==========================================
- Hits        32077    32051      -26     
- Misses       5564     5568       +4     
- Partials     1306     1313       +7     

Files with missing lines	Coverage Δ
xds/internal/xdsclient/xdsresource/filter_chain.go	92.88% <100.00%> (+0.14%)	⬆️
xds/internal/xdsclient/xdsresource/matcher.go	65.80% <100.00%> (+1.65%)	⬆️
xds/internal/resolver/serviceconfig.go	87.07% <80.00%> (+0.63%)	⬆️
xds/internal/resolver/xds_resolver.go	80.78% <46.66%> (+1.30%)	⬆️

... and 17 files with indirect coverage changes