Remove `[[clang::preserve_most]]` (#4319)

These appear to be causing some subtle misinteractions with MSan that we
don't understand, and may be a compiler bug. =/ Fortunately, they
weren't essential to the performance gains so just remove them for now.
When benchmarked on an x86 server, where I would expect this to be more
important due to relatively few named registers, the performance change
appears to be either an improvement or in the noise.

Huge credit to Jon for tracking down that this is related to the MSan
issues.

common/check_internal.h

@@ -37,8 +37,8 @@ namespace Carbon::Internal {
 // `llvm::formatv` which handles all of the formatting of output.
 template <TemplateString Kind, TemplateString File, int Line,
           TemplateString ConditionStr, TemplateString FormatStr, typename... Ts>
-[[noreturn, gnu::cold, clang::noinline, clang::preserve_most]] auto CheckFail(
-    Ts&&... values) -> void {
+[[noreturn, gnu::cold, clang::noinline]] auto CheckFail(Ts&&... values)
+    -> void {
   if constexpr (llvm::StringRef(FormatStr).empty()) {
     // Skip the format string rendering if empty. Note that we don't skip it
     // even if there are no values as we want to have consistent handling of

common/vlog.h

@@ -19,8 +19,8 @@ namespace Carbon::Internal {
 // Internally uses `llvm::formatv` to render the format string with any value
 // arguments, and streams the result to the provided stream.
 template <TemplateString FormatStr, typename... Ts>
-[[clang::noinline, clang::preserve_most]] auto VLogImpl(
-    llvm::raw_ostream* stream, Ts&&... values) -> void {
+[[clang::noinline]] auto VLogImpl(llvm::raw_ostream* stream, Ts&&... values)
+    -> void {
   *stream << llvm::formatv(FormatStr.c_str(), std::forward<Ts>(values)...);
 }