refactor: Use inline constexpr for string literals in headers

This is needed for the next scripted-diff commit, which would otherwise
turn the const from `static const char*` into `inline const char*`.

src/index/blockfilterindex.h

@@ -25,7 +25,7 @@ class BlockFilter;
 class CBlockIndex;
 enum class BlockFilterType : uint8_t;
 
-static const char* const DEFAULT_BLOCKFILTERINDEX = "0";
+inline constexpr const char* DEFAULT_BLOCKFILTERINDEX{"0"};
 
 /** Interval between compact filter checkpoints. See BIP 157. */
 inline constexpr int CFCHECKPT_INTERVAL = 1000;

src/univalue/include/univalue_escapes.h

@@ -5,7 +5,7 @@
 
 #ifndef BITCOIN_UNIVALUE_INCLUDE_UNIVALUE_ESCAPES_H
 #define BITCOIN_UNIVALUE_INCLUDE_UNIVALUE_ESCAPES_H
-static const char *escapes[256] = {
+inline constexpr const char* escapes[256]{
     "\\u0000",
     "\\u0001",
     "\\u0002",