[SanitizerCoverage] Use llvm::getDISubprogram() to get location of the entry basic block.

DebugLoc::getFnDebugLoc() should soon be removed. Also,
getDISubprogram() might become more effective soon and wouldn't need to
scan debug locations at all, if function-level metadata would be emitted
by Clang.

llvm-svn: 239586
1 file changed