Add conditional support for coverage map version 6

This commit augments Swatinem's initial commit in uncommitted PR #90047,
which was a great starting point, but did not fully support LLVM
Coverage Mapping Format version 6.

Version 6 requires adding the compilation directory when file paths are
relative, and since Rustc coverage maps use relative paths, we should
add the expected compilation directory entry.

Note, however, that with the compilation directory, coverage reports
from `llvm-cov show` can now report file names (when the report includes
more than one file) with the full absolute path to the file.

This would be a problem for test results, but the workaround (for the
rust coverage tests) is to include an additional `llvm-cov show`
parameter: `--compilation-dir=.`

src/test/run-make-fulldeps/coverage-llvmir/Makefile

@@ -1,5 +1,11 @@
 # needs-profiler-support
 
+# Rust coverage maps support LLVM Coverage Mapping Format versions 5 and 6,
+# corresponding with LLVM versions 12 and 13, respectively.
+# When upgrading LLVM versions, consider whether to enforce a minimum LLVM
+# version during testing, with an additional directive at the top of this file
+# that sets, for example: `min-llvm-version: 12.0`
+
 -include ../coverage/coverage_tools.mk
 
 BASEDIR=../coverage-llvmir

src/test/run-make-fulldeps/coverage-reports/Makefile

@@ -1,6 +1,12 @@
 # needs-profiler-support
 # ignore-windows-gnu
 
+# Rust coverage maps support LLVM Coverage Mapping Format versions 5 and 6,
+# corresponding with LLVM versions 12 and 13, respectively.
+# When upgrading LLVM versions, consider whether to enforce a minimum LLVM
+# version during testing, with an additional directive at the top of this file
+# that sets, for example: `min-llvm-version: 12.0`
+
 # FIXME(mati865): MinGW GCC miscompiles compiler-rt profiling library but with Clang it works
 # properly. Since we only have GCC on the CI ignore the test for now.
 
@@ -115,6 +121,7 @@ endif
 	"$(LLVM_BIN_DIR)"/llvm-cov show \
 			$(DEBUG_FLAG) \
 			$(LLVM_COV_IGNORE_FILES) \
+			--compilation-dir=. \
 			--Xdemangler="$(RUST_DEMANGLER)" \
 			--show-line-counts-or-regions \
 			--instr-profile="$(TMPDIR)"/$@.profdata \