[sancov][NFC] Make filename Regexes "const"

[nlguillemot]

Summary

The const-correctness of match() was fixed in rL372764, which allows
such static Regex objects to be marked const.

[thopre]

LGTM.

tools/sancov/sancov.cpp

struct FileHeader {
   uint32_t Bitness;
   uint32_t Magic;
 };
 
 static const uint32_t BinCoverageMagic = 0xC0BFFFFF;
 static const uint32_t Bitness32 = 0xFFFFFF32;
 static const uint32_t Bitness64 = 0xFFFFFF64;
 
-static Regex SancovFileRegex("(.*)\\.[0-9]+\\.sancov");
-static Regex SymcovFileRegex(".*\\.symcov");
+static const Regex SancovFileRegex("(.*)\\.[0-9]+\\.sancov");
+static const Regex SymcovFileRegex(".*\\.symcov");
 
 // --------- MAIN DATASTRUCTURES ----------
 
 // Contents of .sancov file: list of coverage point addresses that were
 // executed.
 struct RawCoverage {
   explicit RawCoverage(std::unique_ptr<std::set<uint64_t>> Addrs)
       : Addrs(std::move(Addrs)) {}