[clang] [Driver] Avoid repeated map lookups (NFC) (PR #122625)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-darwin` running on `doug-worker-3` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/23/builds/6556

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM-Unit :: Support/./SupportTests/7/11' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/Volumes/RAMDisk/buildbot-root/x86_64-darwin/build/unittests/Support/./SupportTests-LLVM-Unit-59744-7-11.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=11 GTEST_SHARD_INDEX=7 /Volumes/RAMDisk/buildbot-root/x86_64-darwin/build/unittests/Support/./SupportTests
--

Note: This is test shard 8 of 11.
[==========] Running 126 tests from 82 test suites.
[----------] Global test environment set-up.
[----------] 1 test from AddressRangeTest
[ RUN      ] AddressRangeTest.TestRangesMap
[       OK ] AddressRangeTest.TestRangesMap (0 ms)
[----------] 1 test from AddressRangeTest (0 ms total)

[----------] 1 test from AlignmentTest
[ RUN      ] AlignmentTest.AlignToWithSkew
[       OK ] AlignmentTest.AlignToWithSkew (0 ms)
[----------] 1 test from AlignmentTest (0 ms total)

[----------] 1 test from AllocatorTest
[ RUN      ] AllocatorTest.TestAlignment
[       OK ] AllocatorTest.TestAlignment (0 ms)
[----------] 1 test from AllocatorTest (0 ms total)

[----------] 1 test from CPUArchProfileBuildAttr
[ RUN      ] CPUArchProfileBuildAttr.testBuildAttr
[       OK ] CPUArchProfileBuildAttr.testBuildAttr (0 ms)
[----------] 1 test from CPUArchProfileBuildAttr (0 ms total)

[----------] 1 test from MPBuildAttr
[ RUN      ] MPBuildAttr.testBuildAttr
[       OK ] MPBuildAttr.testBuildAttr (0 ms)
[----------] 1 test from MPBuildAttr (0 ms total)

[----------] 1 test from FPRoundingBuildAttr
[ RUN      ] FPRoundingBuildAttr.testBuildAttr
[       OK ] FPRoundingBuildAttr.testBuildAttr (0 ms)
[----------] 1 test from FPRoundingBuildAttr (0 ms total)

[----------] 1 test from Base64Test
[ RUN      ] Base64Test.Base64
[       OK ] Base64Test.Base64 (0 ms)
[----------] 1 test from Base64Test (0 ms total)

[----------] 2 tests from BinaryStreamTest
[ RUN      ] BinaryStreamTest.StreamReaderBounds
[       OK ] BinaryStreamTest.StreamReaderBounds (0 ms)
[ RUN      ] BinaryStreamTest.StreamWriterPadToAlignment
[       OK ] BinaryStreamTest.StreamWriterPadToAlignment (0 ms)
[----------] 2 tests from BinaryStreamTest (0 ms total)
...

```

</details>

https://github.com/llvm/llvm-project/pull/122625