[libc++abi] NFC: avoid a -Wunused-parameter warning

Summary:
A -Wunused-parameter warning was introduced by patch rG7f0244afa828 [libc++abi] NFC: adding a new parameter base to functions for calculating… (authored by xingxue). The unused parameter base will be used in a follow-on patch D101298. This patch is to avoid the warning before D101298 is landed.

Reviewers: ldionne, sfertile, compnerd, libc++abi

Reviewed by: ldionne

Differential Revision: https://reviews.llvm.org/D104235

libcxxabi/src/cxa_personality.cpp

@@ -245,7 +245,7 @@ readSLEB128(const uint8_t** data)
 /// @returns decoded value
 static
 uintptr_t
-readEncodedPointer(const uint8_t** data, uint8_t encoding, uintptr_t base = 0)
+readEncodedPointer(const uint8_t** data, uint8_t encoding, uintptr_t /*base*/ = 0)
 {
     uintptr_t result = 0;
     if (encoding == DW_EH_PE_omit)