Make sure that -xc++ is always part of the compilation DB. #4974

Job	Run time
CheckFormatAndBuildClean	1m 58s
KytheVerification	0s
ClangTidy	1h 40m 3s
x86_64 test	14m 9s
KytheXrefs	0s
arm64 test	11m 59s
MacOsBuild	1m 49s
📦 Pack Sources	6s
x86_64 test-clang	14m 39s
arm64 test-clang	12m 57s
PrepareVSPlugin	2m 17s
WindowsBuild	32m 50s
x86_64 test-nortti	14m 28s
x86_64 test-c++20	16m 50s
x86_64 test-c++23	17m 31s
x86_64 smoke-test	16m 45s
arm64 smoke-test	17m 13s
x86_64 asan	16m 46s
x86_64 asan-clang	14m 13s
x86_64 compile	10m 11s
arm64 compile	7m 24s
x86_64 compile-clang	7m 55s
arm64 compile-clang	8m 2s
x86_64 compile-static	7m 35s
arm64 compile-static	7m 43s
x86_64 compile-static-clang	7m 37s
arm64 compile-static-clang	7m 36s
x86_64 clean	7m 34s
arm64 clean	7m 20s
Build and deploy GitHub pages	11s
📦 Release	0s
	6h 25m 41s

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