[gcov] Ignore blocks from another file

Constructs such as inline variables, #line, and #include can create
lexical blocks with a different filename.
GCOVProfiling and llvm-cov gcov currently don't handle such cases (see
GCOVLines::writeOut and GCOVFile::readGCNO) and would incorrectly
attribute the line number to the current file.

For now, ignore such blocks. Missing line execution counts is better
than wrong ones.

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As a workaround that Apple targets don't use -mconstructor-aliases yet,
allow line execution count 4 on the A::A line (1f34e282).

(cherry picked from commit 406e81b7)