[llvm-cov][NFC] Add test for coverage-prefix-map remappings

[keith]

Summary

This test covers acts as a regression test for these fixes:

c75a0a1e9dc29be4e00d37d0d00288afc1a6153f
dd388ba3e0b0a5f06565d0bcb6e1aebb5daac065

[zequanwu]

Thanks. LGTM.

llvm/test/tools/llvm-cov/Inputs/coverage_prefix_map/bar.h

+int bar() {
+  return 0;
+}

llvm/test/tools/llvm-cov/Inputs/coverage_prefix_map/main.cc

+#include "bar.h"
+
+int foo() {
+  return 1;
+}
+
+int main() {
+  return bar();
+}

llvm/test/tools/llvm-cov/coverage-prefix-map.test

+# RUN: llvm-cov report -path-equivalence=.,%S/Inputs/coverage_prefix_map \
+# RUN:   -instr-profile %S/Inputs/coverage_prefix_map/main.profdata \
+# RUN:   %S/Inputs/coverage_prefix_map/main.covmapping \
+# RUN:   %S/Inputs/coverage_prefix_map/bar.h \
+# RUN:   | FileCheck --check-prefix=REPORT --implicit-check-not=main.c %s
+
+# RUN: llvm-cov export -format lcov -path-equivalence=.,%S/Inputs/coverage_prefix_map \
+# RUN:   -instr-profile %S/Inputs/coverage_prefix_map/main.profdata \
+# RUN:   %S/Inputs/coverage_prefix_map/main.covmapping \
+# RUN:   %S/Inputs/coverage_prefix_map/bar.h \
+# RUN:   | FileCheck --check-prefix=LCOV --implicit-check-not=main.c %s
+
+# REPORT: {{^}}bar.h{{.*}}
+# REPORT: {{^}}TOTAL{{.*}}100.00%
+
+# LCOV: SF:.{{/|\\+}}bar.h
+# LCOV-NOT: SF
+
+Instructions for regenerating the test:
+# cd %S/Inputs/coverage_prefix_map
+cp -r . /tmp/coverage_prefix_map
+
+clang -fprofile-instr-generate -mllvm -enable-name-compression=false -fcoverage-mapping -fcoverage-prefix-map=$PWD=. main.cc -o main
+LLVM_PROFILE_FILE="main.raw" ./main
+llvm-profdata merge main.raw -o main.profdata
+llvm-cov convert-for-testing ./main -o ./main.covmapping
+rm main main.raw