Add fast paths to CombinePaths and defer path canonicalization in checkSourceFilesBelongToPath

[no-yan]

Part of a series reducing allocation churn on hot paths (see #63895 for
motivation and series-wide measurements).

This PR

CombinePaths built its result through a strings.Builder even when
the last argument is already an absolute path — the common case for
ContainsPath and module resolution callers, where the trailing path
supersedes everything before it. Return it directly instead. Only
slash-rooted paths (without backslashes) take the fast path;
drive-letter and URL-rooted arguments still go through the general
case, which is kept byte-for-byte unchanged as combinePathsSlow and
doubles as the differential oracle for the test.

checkSourceFilesBelongToPath computed the canonical absolute path for
every source file even though it is only used when emitting the error
diagnostic; compute it in the error branch instead.

Results

Two cases covering the fast paths are added to the existing
BenchmarkCombinePaths (benchstat, 8 alternated runs):

Case	Before	After
last argument absolute	44.6 ns, 24 B/op	4.7 ns, 0 B/op (−89%)
single argument, already normalized	33.4 ns, 24 B/op	15.0 ns, 0 B/op (−55%)
six existing relative-path cases	—	no significant change (p ≥ 0.44)

An isolated microbenchmark of the miss path showed ≈+1 ns per call;
at ~125k relative-path calls per full vscode/src check that is ~0.1 ms
total. End-to-end, CombinePaths allocations drop from 185k–207k to
125k per full check.

Verification

• Full vscode/src output is byte-identical to main (43 errors).
• TestCombinePathsMatchesSlowPath exhaustively compares CombinePaths
  against combinePathsSlow (the unmodified pre-fast-path
  implementation) over ~5,800 combinations of representative path
  pieces, including backslashes, UNC, drive letters, URLs, and empties.

🤖 Generated with Claude Code

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