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

Identified with readability-redundant-declaration.

Uses the `TrackingOutputBuffer` to populate the new member `Mangled::m_demangled_info`.

`m_demangled_info` is lazily popluated by `GetDemangledInfo`. To ensure `m_demangled` and `m_demangled_info` are in-sync we clear `m_demangled_info` anytime `m_demangled` is set/cleared.

https://github.com/llvm/llvm-project/pull/131836

Added -p / --no-params flag to skip demangling function parameters
similar to how it is supported by GNU c++filt tool.

There are cases when users want to demangle a large number of symbols in
bulk, for example, at startup, and do not care about function parameters
and overloads at that time. Skipping the demangling of parameter types
led to a measurable improvement in performance. Our users reported about
15% speed up with GNU c++filt and we expect similar results with
llvm-cxxfilt with this patch.

This reverts commit 71f8ea3062a6b0a190835853ee77e58469763b9e.

Test doesn't pass on mac. See comments on
https://github.com/llvm/llvm-project/pull/75348.

Added -p / --no-params flag to skip demangling function parameters
similar to how it is supported by GNU c++filt tool.

There are cases when users want to demangle a large number of symbols in
bulk, for example, at startup, and do not care about function parameters
and overloads at that time. Skipping the demangling of parameter types
led to a measurable improvement in performance. Our users reported about
15% speed up with GNU c++filt and we expect similar results with
llvm-cxxfilt with this patch.

The called std::terminate currently gets the declaration transitively
through llvm/Demangle/Utility.h, removing <exception> from Utility.h
would break ItaniumDemangle.cpp.

To fix expensive check builds that were failing when using MSVC's
std::string_view::iterator::operator*, I added a few expressions like
&*std::string_view::begin.  @nico pointed out that this is literally the
same thing and more clearly expressed as std::string_view::data.

Link: https://github.com/llvm/llvm-project/issues/63740

Reviewed By: #libc_abi, ldionne, philnik, MaskRay

Differential Revision: https://reviews.llvm.org/D154876

D149104 converted llvm::demangle to use std::string_view. Enabling
"expensive checks" (via -DLLVM_ENABLE_EXPENSIVE_CHECKS=ON) causes
lld/test/wasm/why-extract.s to fail. The reason for this is obscure:

Reason #10007 why std::string_view is dangerous:
Consider the following pattern:

  std::string_view s = ...;
  const char *c = s.data();
  std::strlen(c);

Is c a NUL-terminated C style string? It depends; but if it's not then
it's not safe to call std::strlen on the std::string_view::data().
std::string_view::length() should be used instead.

Fixing this fixes the one lone test that caught this.

microsoftDemangle, rustDemangle, and dlangDemangle should get this same
treatment, too. I will do that next.

Reviewed By: MaskRay, efriedma

Differential Revision: https://reviews.llvm.org/D149675

No call sites interpreted this value meaningfully. Simplify this
interface.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D149707

No call sites use these parameters, so drop them.
Equivalent to D148940.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D149703