Move isValidCIdentifier to Strings.cpp.

llvm-svn: 274112
diff --git a/lld/ELF/Strings.h b/lld/ELF/Strings.h
index 1930a0a..002e8c2 100644
--- a/lld/ELF/Strings.h
+++ b/lld/ELF/Strings.h
@@ -17,6 +17,7 @@
 namespace elf {
 bool globMatch(StringRef S, StringRef T);
 std::vector<uint8_t> parseHex(StringRef S);
+bool isValidCIdentifier(StringRef S);
 }
 }