NamedRegionTimer is not thread safe

[llvmbot]

Bugzilla Link	45072
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor

Extended Description

The NamedRegionTimer obtains a timer instance from NamedRegionTimers global,
which keeps a single timer per timer name and timer group name. Those names are
generally constants and a single timer might end up being used from multiple
threads:

Assertion `!Running && "Cannot start a running timer"' failed.

This affects rustc -Z time-llvm-passes when compiling with multiple codegen units.

Downstream bug reports:

• hits llvm assertion "Cannot start a running timer" when timing passes rust-lang/rust#53136
• Debug LLVM + -Z time-llvm-passes asserts compiling simplest program. rust-lang/rust#48329