[llvm-cov gcov] Fix calculating coverage of template functions

Template functions share the same lines in source files, so the common
container of lines' properties cannot be used to calculate the coverage
statistics of individual functions.

> cat tmpl.cpp
template <int N> int test() { return N; }
int main() { return test<1>() + test<2>(); }
> clang++ --coverage tmpl.cpp -o tmpl
> ./tmpl
> llvm-cov gcov tmpl.cpp -f
...
Function '_Z4testILi1EEiv'
Lines executed:100.00% of 1

Function '_Z4testILi2EEiv'
Lines executed:-nan% of 0
...
> llvm-cov-patched gcov tmpl.cpp -f
...
Function '_Z4testILi1EEiv'
Lines executed:100.00% of 1

Function '_Z4testILi2EEiv'
Lines executed:100.00% of 1
...

Differential Revision: https://reviews.llvm.org/D121390