compiletest: allow only running a test if we have a *specific* llvm major version

[jieyouxu]

tests/codegen/try_question_mark_nop.rs has a hack like

//@ revisions: NINETEEN TWENTY
//@[NINETEEN] min-llvm-version: 19
//@[NINETEEN] ignore-llvm-version: 20-99
//@[TWENTY] min-llvm-version: 20

we should allow the test writer to specify a //@ only-llvm-major-version: 19 which will only run the test if the llvm's major version matches 19. Instead of having to hack this by using min-llvm-version + ignore-llvm-version.

[jieyouxu]

Mentoring instructions:

• compiletest: improve robustness of LLVM version handling #132315 should make this easy to implement, should wait for that to land or rebase on top of that.
• Pick a directive name, something like only-llvm-major-version
• Register only-llvm-major-version as a known directive in https://github.com/rust-lang/rust/blob/16422dbd8958179379214e8f43fdb73a06b2eada/src/tools/compiletest/src/directive-list.rs.
• Add a directive matcher in

  rust/src/tools/compiletest/src/header.rs

  Line 1492 in 16422db

  if let Some(actual_version) = config.llvm_version {

  . Check if their major versions match (available as semver::Version after compiletest: improve robustness of LLVM version handling #132315).
• Add a unit test to check that a test would be correctly ignored when major versions match and ignored when they don't in https://github.com/rust-lang/rust/blob/16422dbd8958179379214e8f43fdb73a06b2eada/src/tools/compiletest/src/header/tests.rs.
• (Optional) check existing tests that tries to do the min-llvm-version + ignore-llvm-version version "clamp" and migrate them to use the new exact major version match directive instead.

[LFS6502]

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[Eclips4]

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