[MC] Make UseAssemblerInfoForParsing mostly true

Commit 6c0665e22174d474050e85ca367424f6e02476be
(https://reviews.llvm.org/D45164) enabled certain constant expression
evaluation for `MCObjectStreamer` at parse time (e.g. `.if` directives,
see llvm/test/MC/AsmParser/assembler-expressions.s).

`getUseAssemblerInfoForParsing` was added to make `clang -c` handling
inline assembly similar to `MCAsmStreamer` (e.g. `llvm-mc -filetype=asm`),
where such expression folding (related to
`AttemptToFoldSymbolOffsetDifference`) is unavailable.

I believe this is overly conservative. We can make some parse-time
expression folding work for `clang -c` even if `clang -S` would still
report an error, a MCAsmStreamer issue (we cannot print `.if`
directives) that should not restrict the functionality of
MCObjectStreamer.

```
% cat b.cc
asm(R"(
.pushsection .text,"ax"
.globl _start; _start: ret
.if . -_start == 1
  ret
.endif
.popsection
)");
% gcc -S b.cc && gcc -c b.cc
% clang -S -fno-integrated-as b.cc     # succeeded

% clang -c b.cc     # succeeded with this patch
% clang -S b.cc     # still failed
<inline asm>:4:5: error: expected absolute expression
    4 | .if . -_start == 1
      |     ^
1 error generated.
```

However, removing `getUseAssemblerInfoForParsing` would make
MCDwarfFrameEmitter::Emit (for .eh_frame FDE) slow (~4% compile time
regression for sqlite3.c amalgamation) due to expensive
`AttemptToFoldSymbolOffsetDifference`. For now, make
`UseAssemblerInfoForParsing` false in MCDwarfFrameEmitter::Emit.

Close #62520
Link: https://discourse.llvm.org/t/rfc-clang-assembly-object-equivalence-for-files-with-inline-assembly/78841

Pull Request: https://github.com/llvm/llvm-project/pull/91082

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/MC/MCStreamer.cpp b/llvm/lib/MC/MCStreamer.cpp
index 176d55a..199d865 100644
--- a/llvm/lib/MC/MCStreamer.cpp
+++ b/llvm/lib/MC/MCStreamer.cpp
@@ -93,7 +93,7 @@ void MCTargetStreamer::emitAssignment(MCSymbol *Symbol, const MCExpr *Value) {}
 
 MCStreamer::MCStreamer(MCContext &Ctx)
     : Context(Ctx), CurrentWinFrameInfo(nullptr),
-      CurrentProcWinFrameInfoStartIndex(0), UseAssemblerInfoForParsing(false) {
+      CurrentProcWinFrameInfoStartIndex(0) {
   SectionStack.push_back(std::pair<MCSectionSubPair, MCSectionSubPair>());
 }