[clang-tidy][NFC] Fix readability-make-member-function-const findings

Fix issues found by clang-tidy in clang-tidy source directory.
diff --git a/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp b/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
index ac458d4..0b7887e 100644
--- a/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
+++ b/clang-tools-extra/clang-tidy/altera/StructPackAlignCheck.cpp
@@ -24,7 +24,7 @@
 }
 
 CharUnits
-StructPackAlignCheck::computeRecommendedAlignment(CharUnits MinByteSize) {
+StructPackAlignCheck::computeRecommendedAlignment(CharUnits MinByteSize) const {
   CharUnits NewAlign = CharUnits::fromQuantity(1);
   if (!MinByteSize.isPowerOfTwo()) {
     int MSB = (int)MinByteSize.getQuantity();