[Support] Remove a redundant declaration (NFC) (#166085)

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/Support/StringRef.cpp

@@ -17,11 +17,6 @@
 
 using namespace llvm;
 
-// MSVC emits references to this into the translation units which reference it.
-#ifndef _MSC_VER
-constexpr size_t StringRef::npos;
-#endif
-
 // strncasecmp() is not available on non-POSIX systems, so define an
 // alternative function here.
 static int ascii_strncasecmp(StringRef LHS, StringRef RHS) {